Just my opinion
Dumerials - nice but they are a heavy body snake and do get big even my males are 7-8'
Colombians - the best IMO not as big around as a dumerial but can get 8'
Salmon or Hypos - Most have the temperament of the colombians but way smaller 5-6'
Central Americans - small? sometimes, but not mine and the 30+ I have are not nice.
Nicks - in with the centrals nippy.
Brazilian rainbows - very pretty, they get long but much more slender
I never had rosy boas or sand boas so I can't say

If you were talking to me with your kids I would steer you to a salmon that is about a year old.
The reason for this is because most boas are nippy until they are about a year old. I call it the survival mode. If I stand tall and hiss and strike at you, you will not eat me.