yeah I think a trakpower lipo would be worth more then the truck. Also in the hands of a clueless 7 year old, I don't think so. I'd be having to explain to his mum where his eyebrows went.
As far as tenergy I've never heard of them. I'll look at the shops next time I go.
I found my tenergy cells on ebay, seem to be better batteries than the venoms, IMO. They are sold as various number of cells in packs and loose too. Not sure if you want to make packs or buy prebuilt. Either way they should fit the bill!