We took our son’s family to London for a week when the grandchildren were 2 and 4, and we all had a good time—-Kew was especially fun for the kids, along with riding the Thames Clippers, and the playgrounds my daughter-in-law found (the one is St. James Park was really fun for them). But it could be hot in London in August.
We have also taken our other son and his family to Switzerland when the twin grandchildren were 6, and that was wonderful. We were there during a major heat wave but the alpine villages like Muerren and Wengen enjoy cooler temperatures thanks to the elevation. Wengen has a nice outdoor public pool which might be nice for the children (Muerren has a public pool as well but it is indoors at the Sportzentrum).
We stayed in an apartment in Muerren took the children to playgrounds like these:
https://www.maennlichen.ch/en/summer/experiences/alpine-herdsmans-playground.html
https://schilthorn.ch/26/en/Adventure_playground_Flower_Park
The group could split up and some hike from these places while others stay with the kids. There is always a mountain restaurant adjacent to the playground for lunch. Other activities in the area include boat rides on Lake Thun to a castle, or on Brienzersee to a fairytale “castle” hotel for lunch and a waterfall view:
https://schlossthun.ch/en/
https://www.giessbach.ch/en
Visit Grindelwald in the next valley for a Rodelbahn experience for the kids at the Pfingstegg lift. Kids 4-7 ride with an adult; 8; and up can “drive” their own sled.
https://www.pfingstegg.ch/index.php/en/activities/toboggan
You will also want to visit Luzern, but it can be hot there as well. Consider staying in a nearby village right on the lake, in this lovely little hotel right on the lake, with swimming in the lake or in the pool at their sister hotel next door. You can enjoy a day’s excursion on Rigi by boat and cablecar from this town (or from Luzern if you prefer).
https://www.gotthard-weggis.ch/en/
https://www.rigi.ch/en/experience/families
Your biggest challenge, here or anywhere, will be finding suitable accommodations for that many people. But we traveled with a group of 8 last September and did fine, booking 3 hotel rooms in most places, and an apartment for 6 plus a separate room for 2 in Muerren.