Custom Registration and Shopping Cart

Here’s my situation that I hope someone can help with and/or point me to useful resources:

The site isn’t up yet, but here is what it will be: a main site (like https://www.example.com) and quite a few sub-domain sites (like site1.example.com, site2.example.com) all setup in a WP 3.0 ‘network’. The sites will all revolve around local sports leagues primarily for kids. What will be sold is, basically, seasonal membership in these leagues.

So first a parent will have to register with the site with all kinds of custom fields. Then they’ll find what league they want their kid to be in, and add that to the shopping cart. At that point they would need to be able to enter information for several custom fields (like kid’s name, age, school name, grade, etc.) And if they have more than one kid they can then go and choose another team from one of the sub-sites, and add it to the shopping cart with the same kind of custom information for the particular ‘product’. In other words, the custom fields would be per product, not per cart. Then all that information would be mailed to the site’s owner upon completion of purchase (including all the registration info for the parent).

Can anyone help me here? What Plugins and/or other services I can use to accomplish this?

Thank you!