Why not boas? They are relatively slow growing. By the time they reach adult size your kids would be much older and able to safely be around them. I love the dispositions that my red tail boas have. Most of mine are literally puppy dog tame.

If red tails are still too big, Brazilian Rainbow Boas are a great option. Their care is not much harder than red tails, just need a jump up in humidity. Maybe bull snakes? Easy to care for and relatively cheap. Some can even get to six foot.

Cresties are cool but kinda boring in my opinon.