Can you ask guests if they are actually coming if they do not RSVP by the date on the invite?

The party is for my 9 year old son, but the invitations went to home addresses. Is it rude to ask the parents to tell me if their kids are coming, or should I assume that the kid is NOT coming if I have not heard back? Part of me wants to think that the invitations were not received, but I did get responses from some people....are we just losers?
Can you ask guests if they are actually coming if they do not RSVP by the date on the invite?
Most definitely, it is okay to ask them to rsvp verbally, especially if they didn't already.