I have run into this from time to time traveling with my kids. Best to contact the hotel and ask them (I usually email).

Most of the time it is fine with a child that young - they will say book the room with 2 adults. Once in Dublin it was fine as long as we did not need a rollaway - it was ok as long as we used existing beds or even a hotel provided air mattress.

I have been booking several ALL hotels lately. Many are treating my 12 year old as an adult but certainly not my 5 year old (and even then not affecting price because I am booking those family connecting rooms).