You're not neglecting your child, you're not hurting anyone (except yourself by feeling guilty!) and you are a grown up woman, second marriages are proof themselves that children are not the end of your sex life or your dating!! I would advise against getting ahem down to it with kids in the same room and even with a baby in a crib but only because kids can get a bit confused about the whole thing and it's a lot to explain to a toddler!! With the baby - well, waking them up would kinda put a dampner on things!! Plus I have to say it would feel really weird to me but hey, passion is a good thing if you're in the right location!!
Give it a chance and don't worry - if you're happy and content you have a happy and content child and if there's times he's with yout mother, well, make hay while the sun shines (or roll in the hay as the case may be!)