Uhm.... how Is one of them confirmed a futa now or is that an adopted child?
Could be a family member, a younger brother or something. There's also artificial-insemination, which isn't uncommon for a lesbian couple so they can have a child. Then there's also good 'ol "Science Babies", which is a term Yuri content uses to explain children that can't be logically explained.
I think both of them are too young for a kid that age however. So I'm putting my money on him being a relative.