Hi, this didn't happen to me until I updated the Product-Addons extension. It worked perfectly before this.
http://www.lihoopstars.com - click the Register page, fill in the form, click 'Register' button (is the Add to Cart button with different text) and you'll be redirected to /product/register - what normally happens is the page will refresh with the cart above, this way a parent can register multiple children.
Any ideas?