Wembley has seen a huge amount of development since we were last there reinventing itself from a fairly soul-less part of town into a destination in itself.
https://wembleypark.com/things-to-do...-wembley-park/
https://wembleypark.com/things-to-do...park-outdoors/
I'd also register for free entry to the Boxpark as its inside and very kid friendly especially if you get there early before it gets rowdy.
Go to
https://wembleypark.com/attractions/...signer-outlet/ for a look around the shops / getting something to eat - just don't buy anything as they won't let any bags into the stadium (and they are very strict on this)