5 is an awesome age to travel with. So very many places to go. Spain would be fun. DD loved Seville, but I can't recommend it that time of year unless you are fully prepared for 40 degree (C) days. San Sebastian would be a better bet in June. We did a trip there a few years ago in late June/early July, combining it with the south of France- Biarritz & San Jean de Luz- with a short stint in Pamplona to check out the running of the bulls (kind of nuts with a small child in tow). We didn't feel too rushed covering all of those places in one trip.
Portugal is another possibility. Lisbon is one of my favorite cities and I didn't feel like I missed what I love about it by having DD in tow (she was 4 at the time). The trolleys, especially, are fun for a kid, but there's a lot more to do. I would combine it with a trip to Cascais to get beach time, or you could go a little further and head to the Algarve (which I don't have any experience of). If you don't head to the Algarve, you could go in the other direction and add Porto on to the trip, although between the two cities, I prefer Lisbon.
Spain or Portugal would be great, we love all of spain but a fun trip could be the Galicia region and then down into Portugal to Tui and Porto, stay in Paradores in Spain, they are awesome.
Ireland is also great, or do England and Scotland and drive from London to Edinburgh and spend time in Northumbria.
We took our 2 year old to Lisbon for 5 days ~5years ago. We stayed in Lisbon the whole time, but you could go down to the beaches in the south for some time if you have two weeks. Outside of Lisbon, there are some great sights to see that are only a 30 minute train ride - Sintra and Cascais. You could also combine a trip with Porto.
We stayed in a 2 bedroom apartment and food and wine were ridiculously cheap.