need to refactor child badge price calculation to be in exactly one place

[binary1230]

see discussion in #138

this is not a magprime reg launch blocker

we should try and compute the calculation of "child badge price" in only one location, and not two. that way if we ever decide to change the business logic (which is child price = 1/2 normal price), we don't need to remember to change it in two places.

I am intending to in a future refactor after Jinja2 goes in to rip out the business-logic stuff we're doing in javascript to work around template inheritance not working, so that should get caught there.