Of course each hotel has their own rates/policies for children. Some offer a roll-a-way for no charge. Many hotels have a policy whereby children under a certain age are free if they use the existing bedding, but charge if you request a roll-a-way. Some rooms are not large enough to add a roll-a-way, esp. in Europe. I find I have to search through the hotel's web site to see if they publish their policy. I'll also add that often, the desk person waives the fee for a roll-a-way, presumably because I'm a Platinum member, which is always a nice accommodation.

I just returned from the Westin in Venice... if you look around enough you can find their policy:

Child Policy
If you will be travelling to The Westin Europa & Regina, Venice with children aged 12 and under, one child up to 12 years is welcome to share existing room bedding with parents without charge. Existing bedding does not include pull-out sofas unless a Deluxe Family Room is reserved. Baby Cribs are available at no charge for infants and toddlers aged up to 2 years. Should you require extra bedding for your children and you have not booked one of our Deluxe Family rooms, we'd be happy to prepare a rollaway or pull-out sofa-bed for children aged 3 years and over. A daily supplement of Euro 125 applies. Children aged 13 years and over will be charged as an extra adult and will receive extra bedding.