Thinking of getting an adult or a hatchling?
I have a yearling ghost childrens, my dad got a yearling T+ stimsons. Both are tiny animals (mine was 50cm Dads little guy had slower start and is only 35cm) and this is a big concern for me with my youngest who's 7, he's not the... gentlest/most aware child??? and obviously always supervise, but even with that I worry when he's not paying attention that he'll hurt the little noodle, never intentional just not being as careful as he should. They're very delicate creatures, depends on your child, my 9yr old is a natural at handling snakes and does it deftly without any concern that he's going to hurt it, not an age thing more a personality thing.
Depends on how big an animal you're after full grown also, Anteresia (childrens stimsons spotted) only get 1m to 1.2m bit over as adults, and at fattest part 50c coin. So for me that's what I wanted as first snake (I know more are to come in future now that I've given in to it), to grow with the kids. Dad got his too knowing that it will one day end up at my house as he is older in years and won't be around for whole life of the snake. If you want something bit bigger when it's older go a woma or blackheaded, or carpet of one kind or another, you can get them as a hatchling/yearling and handle it up lots and it'll grow with your child and start at a more... robust size?
Again it all depends on your situation, your child's interest.... I assume realistically that the snake will be yours. So get what YOU are interested in and able to confidently handle, get it young so can handle it up loads and be happy with it, your grandson can handle it too and grow in confidence as it grows etc.