This is a difficult one and it does depend on the hotels, a number of them will have a policy like the Shangri La hotel in Colombo that has children's policies:
"Children of registered hotel guests under the age of 6 can enjoy *buffet meals at no extra cost in the All Day Dining venue when accompanied by a paying adult; up to 2 children. For more than 2 children under the age of 6 or children who are 6 years and above but under the age of 12, a 50% discount on the buffet price will be given".
However, not all hotels will have children's policies in place so is it always wise to ask before booking.