I find that when making reservations, especially in Europe, sometimes I have difficulty putting 3 guests in a room. It will just say that they have no rooms that can accomodate 3 guests. So I've just always changed it to 2 and so far I haven't ran into any trouble. But, my daughter is only 7 and she is still small enough to squeeze into a queen or king bed if it's a single room.
So, I was just wondering when I should expect that I should start making sure to count her separately.
The problem will be that at some point when you show up at a hotel and the front desk sees that there are three of you arriving for a room reserved for two guests, the front desk may decide for you that she is too big and must be counted as her own bed occupying guest. It's a judgement call. What you and your wife think about her size and whether or not she can comfortably share a bed with you may greatly differ from the hotel staff and they ultimately get to make the call.
Think about some of the room sizes that you've stayed in (in Europe). Some rooms, whether a king/queen or two twins are so small that there is no possible way that you could squeeze a roll away bed in without creating a fire/safety hazard (and many hotels have strict policies and will simply not do it). I can think of a number of rooms at Marriott properties in Europe where this would apply. Once, we (three adults plus a toddler and a baby) reserved a junior suite at a Paris Renaissance. It was a good sized room and the thought was that the two adults could sleep in the king bed, one adult could sleep in the sofa bed with the toddler (age 2) and the baby could sleep in the port-a-crib (pack n' play). Seemed reasonable. When I made the reservation, I plugged in 3 guests to get the room to show up (if I had plugged in 5 guests, nothing would've shown up except the Vacation Club.) When we arrived, the folks at the front desk looked at our party with eyes as big as saucers and almost didn't give us the room, citing fire/safety issues. They gave us the room in the end, but were very reluctant (and in our case, quite different from yours), we ended up securing a second room after the first night (costing an extra $750, but worth it.)
I would say that at some point (sooner rather than later) in your case and depending on the size of your little girl, that in the absence of you deciding to add her to the number of guests in your reservation, the hotel will make the decision for you as far as determining how many will fit in a room reserved for two. This can get tricky. You just wouldn't want to find yourself facing the unpleasant surprise of having the hotel deny you your room or else telling you that you will have to take two rooms if you want to execute your reservation.
Just my 2 cents based on my own experiences.
We never use a rollaway. I typically try to get a room with two double beds or else a queen or king.
I guess I just don't see what they expect us to do. If they claim you can only have 2 guests in a room it's not like I can put her in a room by herself.
One place did charge us extra for her breakfast, but that wasn't that big a deal.
I know at some point, as you said, she will be too big/old. Trying to pinpoint when that is part of the reason I posted the question. I've seen a very few hotels that specified ages, but the vast majority never mention it.
this is always an issue I look into before booking.
what I have found is most of the marriott hotels I have stayed at are fine when the kids are under 12.
it often states in the room conditions that kids under 12 stay free in existing bedding and no charge for breakfast.
also if we request a rollaway I do check if there is a charge.
if I am in doubt I ask the hotel if there any additional charges for a child in the room.
I think Marriott has pretty much defined a kid as under 12 for the purposes of their "kids eat free" policy but numbers in rooms is very different since those policies are set by local fire and safety regs which vary not merely from country to country, but state to state and sometimes even town-to-town. My method of checking is to find out if rollaways are permitted. If so then I take the view an extra kid can join us. If not then its 2 max for a king, 4 max for a double-double...
I usually count my children as soon as they're born.
IME the hotels never seem to care about 2 or 3 or whatever people. The only difference is if your rate gets "free breakfast". If you book for 2 and 3 go to breakfast they catch that and bill you for 1 meal. But I've never been questioned as to number of people actually staying there.