How long has your youngest son had his corn snake? Also, how old are your children? Four snakes require a lot of care, expense to feed them, vet bills-- no matter how you house them. Do all of your children sincerely desire a pet snake, or is there a novelty factor to consider here? They want them today, but will they clean the enclosures tomorrow?
I keep 20 snakes, plus a bearded dragon. My son kept a colony of six leopard geckos and a group of six golden geckos for many years. A little over a year ago he enlisted in the army, leaving me with his legged reptiles. Taking care of them is added work. However, when I let him get the lizards, I knew that someday they would probably become my responsibility.
Captive bred snakes can live 10 years with good care. Responsible keeping is making a commitment to care for a creature for it's lifetime. This means not only daily care, but veterinary care when needed, etc.. Are you, and your children, prepared for this?
I don't mean to discourage you. It's just that there are issues to consider beyond how many snakes to keep in an enclosure. You might consider adding one more snake in it's own enclosure, with a care schedule for the children. In six months time it will become clear which children really are mature and interested enough to be junior herp keepers.
My son acquired his geckos over a span of years, as he proved to me that he was responsible enough to care for more of them, breed them, etc.. It was a rewarding, positive life experience for him. I wish your children the same type of experiences!
rgds,
althea