photoman43

Here is one person's list of best glass for Nikon bodies: All will be expensive. See unnumbered pages 26-27.

https://ejphoto.com/Quack%20PDF/Quack%20Winter%202020.pdf

hogilbert

I would choose the 50mm. I recently took portrait (waist up) of seniors sitting on Santa's lap. with a D750, Nikkor 50mm 1.8 D and they were all nice. If you can find a used f1.4 or 1.8 D lens you will be happy. I find that the older D lens is sharper than the newer G, I have both.

Photocraig

Especially for indoor portraits and candid shots, I'd start with the 50mm f1.8. I've seen great results from the Tamron 45mm, but it is big and pricey. Perhaps a future buy for you if you like the "normal" focal length and want a better quality image with "VR."
The 50 will give good perspective for faces without the distortion of a wider lens. Since you have many Megapixels, you can easily crop the candid photos to fill your frame or print with the kids' (smaller) faces, and still not sacrifice much quality. I think you will like the compact almost stealthy (OK like maybe like a big Bridge Camera) and mobility the 50mm will give you.
As a Photo Skills development tool, using a single focal length for a variety of subjects and outings is a good exercise. The 50mm gives you a chance to explore "seeing" different photos within an overall scene or place. AND, never underestimate the low light opportunities,or high shutter speed to counteract hand shake that the f1.8 maximum aperture gives you. Also the freedom to shoot in, almost, the dark or blur the background to separate a subject from the (distracting??) background.

The relatively low cost and versatility of the 50mm, largely due to the high volume produced, and the small size and weight make this close to a "no brainer" for a Full Frame kit. When the budget is full again, and your experience is greater, I'd suggest looking for a gently used 85mm from KEH, B&H, Adorama, Cameta or Roberts. That longer focal length provides a nice formal, as in posed, portrait perspective and the mild telephoto mildly compresses facial features for, some say--me included, a pleasing look.

A Holiday Photo tip: Be sure you set your White Balance correctly depending on your indoor lighting. Today's homes have a mix of fluorescent, LED and incandescent lighting. Nothing makes a person look more ghastly that the wrong White Balance affecting the facial skin coloring. If your shoot in JPEG only this is essential. A solution is to shoot in RAW + JPEG, and use the RAW image in a Post Processing Program, the one that came with your camera is good for this, and adjust White Balance and colors easily with sliders, there. But, that's an entirely different skill level that you haven't mentioned.

Merry Christmas,
C

bwana Loc: Bergen, Alberta, Canada

50mm and 85mn are different enough that you NEED both! Or find a zoom that covers that range...

bwa

DeanS Loc: Capital City area of North Carolina

If you have a 20-70, you sorta have the bases covered. However, if you decide on one, the 85 will be sorta tight if most shots are indoors. That being the case, opt for the 50.

Angmo

Most 50s aren’t that great.

Here’s a competent review and also read up on recommended lenses for the D850

http://www.dslrbodies.com/lenses/nikon-lens-reviews/

If I could chose only two, 35 & 85

Well and the new 105 1.4

buckscop Loc: Bucks County PA

I have both and find the 85 hard to use at home indoors as it is to close and I have back up quite a bit, or forget the shot. I love the 85, but if only 1, and in-home shooting, go 50.

Saxguy Loc: Plano, TX

Congrats on your D850! I find mine to be awesome. I use a Tamron 85mm for portrait lenses and the two are a marriage made in Heaven.

Iabrams3985

The 24-85 mm Zoom-Nikkor is worth considering; inexpensive and quite sharp. I use mine on my D850 with plenty of sharpness.

Allan Castle

Just use your 24-70 for most photos. Being able to zoom is very helpful for candid shots of kids. Both of your zooms are sharp and in low light your D850 will be good to high ISO's like 3200 and even higher. The main advantage of the prime lenses is lens speed and bring lighter in weight.

Silverrails

Is D850 a DX or FX camera? I myself have a used D3300, and I chose a 50mm 1.8g Prime as my 1st lens, I now have a 18-140mm, a 35mm 1.8g Prime, just ordered the Nikon 10-20mm 4.5-5.6 wide angle.

CO

I have that lens for my Nikons. Not many prime lenses have stabilization. Its vibration reduction is great as it gives about 3 to 3.5 stops of additional hand holding ability. It's built to professional standards with a metal lens barrel and extensive weather sealing.

MoT Loc: Barrington, IL

It sounds like you need a lens that will satisfy your need for group and individual shots. Since a zoom is not in your plans (24-80mm or so f2.8) would be the best but if your budget is limited to one prime than I suggest the 50 mm f1.8 which would allow you to get group shots and portraits. That said in order to get close enough for portraits you will have to be quick as kids tend to scatter when you get too close with a camera. I have been most successful with a zoom and have found having a longer lens (135-200mm) has allowed me to get great shots at a distance so the kids are not aware and you catch them in their normal state. All that said for your needs the 50 mm is the way to go. Also as f1.8 or wider apertures allow you to get available light shots ( which I prefer) as you did not mention if you have a flash.

digit-up Loc: Flushing, Michigan

I was quite impressed with it, and I don’t think that it was terribly costly........RJM

ralf Loc: NJ

My preference for a single high-quality lens to go with your high-quality D850 would be the Sigma 50mm f/1.4 ART lens. It is not cheap, but it is a killer lens. I love it. It is big and heavy compared to other 50mm's -- which is the price you pay for f/1.4 I guess. You can do portraits with it, and low light is way less of an issue for f/1.4. To my thinking the 85mm is too telephoto -- altho many people like 85mm for portraits.
Personally, I am a big fan of non-zoom lenses -- you give up the convenience of a zoom, but you get speed and sharpness -- which seems like a good trade-off to me.