Am I right in thinking that we can care for 6 children under the age of 8, (3 under 5's) so once a child turns 8 we can care for more than 6 children in total?

Yes you can and for insurance purposes the maximum is usually 12 all together - 6 under 8 and 6 over, but be careful because Ofsted don't like a lot of older ones if you have under 5's and also it depends on your space (area) in your house.

It also depends on your LA Planning Dept. Some areas allow 6 under 8s and have no restrictions on over 8s, in my area I can only care for 6 children of any age. Some areas include your own children others don't.

This thread has prompted a question: has anyone ever been inspected when school children are actually in our care or are all inspections just in the morning with the EYFS children?
Interesting to know...