First preference:
Whatever makes you happy!
We do this for the fun of it, you know.
Second preference:
Your current lenses with either an SB-400 or SB-700 and an SC-29 remote cord to get the flash off-camera.
Grandkids never stay still and a flash freezes motion far better than any shutter speed your camera can achieve.
The SB-400 has a tilt head and recycles quickly.
The SB-700 tilts, swivels and zooms, can light up a basketball court and recycles quickly. It also comes with filters to correct color balance for incandescent and fluorescent lighting.
Current Nikons do a fabulous job of balancing fill flash and ambient light for you, so you can focus on your subject.
Point the flash in the direction of their faces with a slight bounce upwards, holding it overhead and to the side to eliminate any outline shadows.
Cost is about the same as one of the lenses you are considering.
Third preference:
50mm f/1.8 or f/1.4. The f/1.8 is a lot cheaper and both are excellent lenses.
The key to portraiture is not focal length, it is camera-to-subject distance.
For a good head shot with a 50mm, stand as far away as you would with an 85mm and crop the photo for head-only composition.
With 45MP (sheesh), IQ will NOT be a problem.
My recommendation, with your situation:
Wide-to-tele zoom, SB-400 on-camera and bounce flash.
Small, light and effective.
But, since you would have to wait and find an SB-400 used, the SB-700 would be the best "right now" solution, buying from your box store.
It works perfectly well on-camera, bouncing the light to eliminate shadows.
It won't give you the modeling you can get off-camera, but it will get you great pictures.
Merry Christmas and Happy Shooting!
P.S. This is the only way we can get away with shooting our grandkids and not end up in jail.
Just sayin'.