Yes it is, but phone and check with the individual hotel before you book.
If your 2 year old still sleeps in a cot it shouldn't be a problem at all. One Premier Inn actually suggested to us when DD was just about getting too big for a cot that they would let her top tail with her brother so that we could still fit 3 DC in. Not every hotel does, but if you check first you should find one.
JM is right it is much easier to do this when they are 2 or less and in a cot because then you can just put a cot in a room. Also pays to be up front rather than get there and find that they won't let you do it or make you pay tops for another room. Once beyond this we have found it near impossible to smuggle another child in so suggest you start looking at the take5 type websites in preparation! It does seem to be easier to find for 5 now than a couple of years ago so maybe they are learning that families arent just 2+2
I think the problem about fitting huge numbers of people into one bedroom is to do with fire regulations. So please make sure you are able to do this. If you do sneak in an extra child when you aren't supposed to some jobsworth might report them and the staff will get into trouble.