Well at least you could just tell them you are basing the decision strictly on who was there first, so it isn't personal. Just tell them you only want to have four full-timers and because the original kids were there first, you had to give them priority. That way if they ever bump into you out with the kids, they won't think you lied to them (if you don't tell them you are keeping some kids) and no feelings will be hurt. Good luck!