Then there's no way I'd suggest using a lone sponge filter for any long term success in that tank what-so-ever. With the amount of bio-load that tank will be experiencing, combined with their lack of fish keeping experience (my assumption, but I doubt I'm that far off base), you're going to want to crank up that filtration by quite a few notches. Sponge filters are meant for fry or quarantine tanks - not for what you're looking for...
If I were you, I'd skip the sponge and go and get a pair of AquaClear 110s for them and start there. If they're up to it, the addition of a solid canister filter down the road would be good as well.
Good luck!
PS - Consider it your duty as a fish hobbyist to teach them how to fish, rather than just giving them fish (if you know what I mean). A good investment for you to give to your parents would be the book The Simple Guide to Freshwater Aquariums by Boruchowitz. It can be found for next to nothing on Amazon.com and is what I consider a must read when it comes to starting out in this hobby. I did the same thing for my parents and they couldn't be any happier with their 75G African Cichlid community that they put together pretty much by themselves.