My husband and I took our then 4 year old and 1 year old to Dublin a couple of years ago. They had a wonderful time! The LUAS trains were exciting for a couple of train-mad boys, and the LUAS took us straight to St Stephen's Green and Grafton Street to start our explorations.

The playground at the heart of St. Stephen's Green was always full of young children from all over the world, speaking numerous languages, so the kids had the multi-lingual experience with kids their own age. It didn't seem to matter to any of them.

Grafton Street had wonderful street performers that fascinated the kids, especially when they recognized one of them from the train ride in!

My best recommendation, though, is to pick where you wish to go, then find a guide for that city that has a good children's section in it. I've usually found Rick Steve's and Frommers to have the best and most practical suggestions. YMMV. Find as current a guide as you can, but also check the used bookshops or library sales for older guides. While the prices and/or hours may be out of date, most of the really good attractions don't vary all that much, and you can at least get some good suggestions from them.

Have a great time, wherever you go!