Is it permissible to bring small kids, provided they are well-behaved, into the confessional while a parent confesses?
No. The seal of the confession is absolute. Not only does it bind the priest but it also binds anyone who overhears the confession. You cannot put a burden like that on a child. You might coordinate with other parents at the church to provide babysitting during confession times. If that’s not possible, then you’ll either have to bring along another adult to watch the kids and trade off sitting responsibilities while the other confesses, or find a babysitter to watch your children while you go to church to confess.