The type of room we book depends on the information each hilton gives in the room description and/or family policy which often pops up when you add kids to the booking. The problem seems to be inconsistency throughout brands and countries. In the USA, we have never had a problem booking the kids in a standard room and this is usually clearly described on either the room description or in the family policy.. Outside the USA it really varies and sometimes I can't find a family policy or any info on kids in existing bedding/extra beds. In this case, we make sure at least one of the kids is included in the booking (in an extra bed) and then add that we will have an additional child in existing bedding in the notes. This has worked well for us without comment until just a few days ago in Zurich, when it was questioned and resolved by a free upgrade.
if in doubt you could always email the property in advance and ask if you are concerned.