this discussion is very relevant. My10 year old granddaughter expressed an interest in creating photos. She and her siblings and cousins have grown up with a camera as I never lose an opportunity to photograph these precious treasures.
For a 10 year old, a bridge camera is the correct purchase. I guarantee that most 10 year olds would not acclimate to interchangeable lens set up. ‘Now where did I leave that lens.....?’ Focal lengths and f/stops are confusing enough on a bridge set up. I purchased a new Nikon bridge camera. It’s light, easy to use as a p&s until she understands better the opportunities resident in that camera. “Grandpa” does not indicate Metusaleh. The term is to be used by grandchildren addressing their grandparents. It is not a moniker for you out there to label him. I have no illusions with respect to the camera gift. She has an obligation to use it as she asked and knew that this gift, although given as a Covid 19 salve, is more expensive than the norm. It is also a wonderful opportunity - for me, for her, the artistic community (don’t sell anything short!)!,

I love my 3 canons, xsi, t3i, 77d. if I was looking at them I would get one without the articulating back so you can view from all angles. I think the t3i is the first model that did that. lot of new older model canons are very reasonable now. I've never bought used but the hogs can advise you more.
interchangeable lens are great way to go. i have a 24 & 40 Canon and they are so light weight you might consider one with the kit 55 250 and she has a light weight option. shes ten now so big lens might be too heavy but with the kits they don't add much cost so get it anyway. I got my 24 mm for 100.