I would suggest Dublin so your child will understand what people are saying. While it is true that most people these days in big cities at least understand if not speak English, still I think it iwll be easier for your child if he can understand the everyday language and ask questions that will be understood by everyone.

As an American who lives most of the year in Europe, I have been to all three suggested locations, and while they are all great, I love Dublin. It is pretty and the people are incredibly friendly.

Jaelle