Book recommendation for 7 year old
There are at least 2 books that come to mind:

National Geographic Readers: Rocks and Minerals (2012), a richly illustrated guide for kids.
Ultimate Explorer Field Guide: Rocks and Minerals (2016) by Nancy Honovich, a guide focussed on getting kids outside, looking at rocks in their backyards and on the camping.

Considering you're in the UK, afterwards I'd start with some experiments about limestone. National Geographic used to have rock kits for kids, not sure who else sells them near you. With the keywords &quot;rock kit kids uk&quot; I get some results that may be worth a look at.