Porto works with children if you treat the hills as part of the plan, not a surprise. I would not book three timed attractions in one day; I would pick one indoor stop, one place to run around and leave room for a snack detour.
For a rainy morning
SEA LIFE Porto is the straightforward choice for younger children who like tanks and sea life. It is beside Parque da Cidade, so if the weather clears you can walk outside afterwards. Check the official ticket page for entry times and feeding sessions before promising a particular one.
World of Discoveries near Alfândega is an interactive history attraction with a boat ride. I would choose it for children who enjoy theatrical sets and stories; older kids who want a conventional history museum may prefer something else. Check the current visit details before you book.
For space to move
Parque da Cidade is my reset button when everyone has had enough of narrow streets. It reaches the coast, so you can walk toward the beach if the weather is kind. Closer to the centre, Jardins do Palácio de Cristal give you paths, peacocks and river views without leaving central Porto.
Keep lunch easy
Mercado Bom Sucesso is a low-friction family meal: several counters in one place, so people can choose different food without turning lunch into a negotiation with a long menu. For something very central, Bolhão is lively and useful for a snack, but remember the market is closed Sunday.
Getting around with small legs
Walking downhill into Ribeira feels easy; the return up does not. Use the Metro or a taxi for the long leg and save energy for the part you actually want to see. Pushchairs are awkward on old cobbles, narrow pavements and stairs. A carrier can help, but do not plan a full day of carrying a child and a bag up Porto’s hills unless you know you can manage it.
For a first day, I would do São Bento, one short look at the river, lunch and a park. Put SEA LIFE or World of Discoveries on a separate half-day. Check current child fares and ticket rules with Andante; do not guess from an old blog post. Bring snacks, plan a toilet stop before it becomes urgent, and call the day when the children are done. There is no prize for reaching the next viewpoint with everyone miserable.
