little league

hi everyone! i have just been tasked to add commerce capabilities to an existing D7 site. i need to add the capability for parents to pay online for their children's little league registrations. has anyone done anything similar and does anyone have any recommendations.

basically the requirements include:

• i need the parents to be able to register on to many children (i can limit the number to 5 or 6 since no one has that many if it makes it easier)
• the player registration costs drop 20ドル for each child (i.e. 1st child 140,ドル 2nd 120,ドル 3rd 100ドル and so forth)