Who’s just at 50 pounds and 4’ tall? I am thinking an XS Tipp Ride Lite, but have heard they are pretty flimsy? Any other options that come that small?
I don’t know much about how the sizing runs-- and you will probably need to take measurements anyway-- but I know Rodney Powell, Airowear,Charles Owen, etc all make children’s vests.
I’d also suggest checking ebay for Intec Flex Rider vests - the older solid sort, rather than the Tipperary knock-offs. My kids have worn these, and they made one small enough for my son when he was 5 years old. They’re still floating around on the internet and can be a good bargain. This one’s a medium, but it’s the same type: http://www.ebay.com/itm/Equestrian-Safety-Riding-Vest-Child-M-/142100672938?hash=item2115dc31aa:g:13UAAOSwGtRXw16H .
This might be more info than you want, but here’s a Horse Journal article on vests, and it lists some that have child sizes as well. http://horse-journal.com/article/heres-choose-body-protector-25128
Ladies size XS is definitely still too big for a 7 year old. Think you have to aim for something sized specifically for a kid.
Yes, I meant an XS Childs as he’s pretty small 
Check the UK Ebay site?
I’ve done that to look for foxhunting gear (with success) and as safety vests on all the tiny riders are just how it is done there, you might have more luck finding a variety of options.
