Porto or north eastern Spain

Time of year?

I assume , starting in Porto, you would tour within the rest of Portugal, at least to Lisbon? It is a wonderful small country with friendly people. The Minho region north of Porto was a favorite of ours.
After we finished visiting Portugal, we took a taxi across the border with Spain, picked up a Spanish rental car, toured Galicia, Asturias, Cantabria and the Basque region of Spain. If you do visit the Basque area, I recommend that you read The “Basque History of the World” by Michael Kurlansky.
Bilbao to Hondarribia, the Spanish Basque region
offers a very different culture, food, and architecture. Would you drive to Barcelona, stopping along the way? Or fly or take trains there?
Barcelona( Catalonia) and the Basque area support each other in their quest to keep their regional cultures and languages alive. Barcelona has beach access, lots of interesting architecture but is a huge urban area. These are very different places.
You could also spend some time on the beaches of the Costa Brava north of Barcelona on the coast.

Porto and the Douro River Valley are amazing. We did a seven day river cruise that was fantastic. If you don't want to do the cruise, take the train up the river to Regua and more.

We have not visited Barcelona with children so I can't comment on that part of your question. We have visited Portugal many times and ten years ago we visited with our family. Our granddaughters were 8 and 10, so about the same age as your children. We did not go to Porto with them but visited other towns. With two weeks you could travel between Porto and Lisbon, visiting some small towns in between. Keeping in mind the energy of children, we visited the Roman ruins of Conimbriga. They enjoyed walking/running along the paths and of course, the adults enjoyed the history. This is also not far from Coimbra. The children enjoyed the university and some of the steep streets. Closer to Lisbon, they liked climbing the Tower of Belem, the Coach Museum and Castelo Sao Jorge, which is mainly grounds where the children can be as energetic as they like. They also enjoyed Sintra. Pena Palace, the outside but they weren't interested in the interior of the Palace. They also were able to hike around the grounds with their parents. There is a Toy Museum in Sintra which everyone also enjoyed. We spent a few days in Nazare, which some on this forum don't appreciate, but we enjoy it. It's on the sea and you can also take a funicular up to the town of Sitio for great views. The boys would probably enjoy the ride and walking along the beach in Nazare. From Nazare you can take a couple of day trips to Obidos and some of the many monasteries and other sights in the area. We also visited Castelo de Almourol on a trip without children. It's set on an island in the middle of a river. Supposedly they only way to get to the castelo is by a short boat ride but we didn't see the sign for the boat but it must have been low tide as we found some rocks that we could step on to get to the castle. You and the children might enjoy this. Of course, the adults enjoyed all the sights and activities as well as the children! I'm not sure if this is the kind of information you're looking for but I hope it's a bit helpful.

Hopefully someone will have someone will have some child friendly suggestions for Spain.

Have fun with your planning.