Matilda

Throughout August there is 'kids week' where 1 child goes free with each paying adult.
The tickets go on sale in June. You don't know what musicals are going to be offered in advance, so you'd need to be quick and no guarantee that Matilda would be amongst them, but if you're prepared to risk it's an option.

You might be lucky because Matilda is also touring. I went to see Wicked a couple of years ago with my DD through kids week and we could get tickets easily because it was also touring so more people going to local venues. You could also look at that, if it's coming to somewhere close to you. Tickets might not be much cheaper, but you might reduce cost of travel and other expenses, depending on where you live.

Link to kids week

https://officiallondontheatre.com/kids-week/

Link to Matilda touring

http://uk.matildathemusical.com/tour/