I would recommend purchasing the green screen movie package here
which would include a moderately sized green screen (5X7 I believe), Sony Vegas Platinum, and the FXHome Composite Lab Pro program. Additionally, you could purchase the Composite Lab
program separately if you already have a green screen or editing software (NLE) such as Premiere, Vegas, Final Cut, etc.
If you plan to seperately purchase a green screen, local fabric shops will often have usable fabrics at reasonable prices. Also, visiting our friends at www.tubetape.com
will yield some great products, and they ship quite fast.
Just for reference, Vision Lab Studio (pro licensed) would provide green screening abilities in addition to stunning visual effects such as those in Effects Lab, but Composite Lab may be better suited for your needs and it's a bit less expensive.
I hope that helps, it's always great to see kids getting involved and enjoying the software. Best of luck.