Hello,
I would also depend on how old the two children are.
If they are old enough that you no longer have to check on them every night (possibly multiple times), I would also put the children's rooms on the 1st floor and the parents' area under the roof.
Especially in the teenage / young adult age, I would find it more practical if a boyfriend or girlfriend doesn't always have to "sneak past" the parents' bedroom :D. But in my opinion, the parents' privacy is also better maintained this way.
Regards,
Dirk