Yes, even the simplest trip with DT to soft play is £30+
£6 for each to get in, plus me, totals £14.
I'll take drinks and a snack, but they'll inevitably see other children eating the lunches/cakes/ice cream. £2 each for a cake. £6 each for a lunch. It's no fun for them to always hear no, when they see other children enjoying the food. I don't let them have the ice cream though. Then I'll have several cups of tea at £2 a go.
That's £36 already. Factoring in a third child you'd have, makes it £50.
Yes, there's free things to do. Walks. But only you know if you're happy only ever doing the free things, and won't feel sad that your DC only get a paid trip occasionally because it's so expensive with 3. This becomes more apparent when their peers with higher income families/no siblings/fewer siblings start talking about this weekend at the zoo, and the weekend before at the trampoline park, and the weekend before at the festival, and the summer holidays starting and they're going to a waterpark in Spain/Disney/whatever. And the other child can only repeat "I did some playing, went for a walk, and went to nanny's" 90% of the time. I don't believe in spoiling DC at all. But I do believe in them being able to experience as much of the world as possible, including the country they live in.
I'd have loved a 4th. It would not have been fair to the 3 I have, to have another.