Take a look at this page http://en.community.dell.com/support-forums/desktop/f/3514/t/19423852.aspx
That shows all the cpu upgrades for your SMFF and motherboard...you can look up the prices for yourself, (you didn't mention what size your budget was) and then decide what fits your budget and what you feel like tackling with your kids!!
Be sure ram and cpu are compatible as well!
Good Luck and enjoy the fun of working with your children on a technology project!