I don't think polite is going to work. Say whatever you want to say!!

Personally, I would say yes, i am a first time mum & i'm doing things the way I think/feel they should be done. I'll bet you were the same with your first!

As for being around others sick kids, it's not them that has to deal with it if your baby gets sick, so they don't actually think about it. Just a simple "I don't want my baby or myself getting sick, i'm not going to put us at risk if I can avoid it!"

HTH!