DD is 15 months and has been constipated for about a week. she is trying to poo frequently but often not much is produced, and when she does it's very hard and dry and takes a major effort. her tummy is pretty swollen 
Usually a spoonful of prune purée in her porridge will get things back to normal but this time it's not helping.
Any other ideas? When is it time to go to the GP or get some over the counter laxatives (I don't like the idea but on the other hand I really don't want it to turn into impaction or withholding) assuming such things are available for toddlers...
thanks!