Places that won't be very very busy in London next week, suitable for a 7yr old with ASD, moon on a stick?

[Catoninetails]

My teenager is desperate to go to London for an art exhibition they want to see before it finishes at the end of August. This is doable as we have family we could stay with who would love to see us all, but I'd also have my 7 yr old who has ASD/ADHD with me if I go. I could send teenager alone but she hasn't done the journey before and isn't confident about the underground etc, plus it would be nice to see family so I'd like us all to go if possible.

Is there anything at all that won't be packed that I could do with my 7yr old? Likes include Minecraft, history, space, dogs 😂, Harry Potter... But very busy places are a real struggle as are long queues, so am I being unrealistic?

Went recently with my little Asd 9 year old. Our nicest day was a riverboat to North Greenwich and a cable car ride over the river. Also have a mini museum and plane simulator. Once you’ve crossed the in the cable car there’s a paddling pool which is great on warm days like we had. Almost no queues at all throughout the day.