Hey,
I would have thought that if it was fostering and not straight-up adoption, that you can negotiate an agreement where the kids can still know about you.
I would think it was easier to do that with fostering than adoption, where it's common to have no-contact adoptions. Fostering is more open-ended, no?
PS I think it's great that you want to still be involved in these kids lives, to make sure they get the best, despite not being biologically linked to either of them.