What I usually do is try to find the family policy. Sometimes this pops up when you select 2 adults and 1 child and sometimes you have to click on the hotels main website and look under hotel policies. Also, sometimes it is not shown or listed and in such cases I usually email the hotel directly.

I booked many hilton standard rooms firstly with 1 child and now with 2 (often the rules are 2 children under 12 in existing bedding, but usually they have extra beds anyway). If it was a worst case scenario, you could have a twin and the child stays with mum or dad. One of the problems of hilton.com is you cant select the age of the child, so when you select child 0-17 it might say no, although the room description says something else. If you give us the name of the hotel, maybe someone has some experience and can advise more accurately.