How about Barcelona. Has a beach in the city, lots to see, very child friendly. Great food. You should be able to rent an apartment in the city centre and Aer lingus fly direct. We took a baby about the same age, with 2 older children, a couple of years ago. It was great. The kids love the variety and there is always somewhere close at hand to sit, and/or eat. There is plenty of culture for adults and the buses are a great way to get around. We put in 2 weeks no problem, the kids got a great reception everywhere, even in posh restaurants, one waiter went out to the corner shop to buy them ice lollies because the deserts on the menu didn't appeal to the kids.