It's on Mercer's Row, Fimbo - if you drive out of Cambridge on the Newmarket Road, you'll find there are signs for Garlic Row industrial estate on the left (shortly after B&Q). Go down Garlic Row and Mercer's Row is the first right. I haven't been yet, be interested to know what you make of it.
Anyhoooo, further family friendly ideas for Cambridge include:
Picnic on top of Castle Hill (for a view - something you might be gagging for after a couple of days in town)
The classic punt thing is to punt up to Grantchester, have tea at the Old Orchard and back - beware, this is a long punt for those inexperienced and likely to lead to a family row after the first pleasant half hour!
Tourist bus (DS seemed to enjoy it, anyway)
Cambridge Model Railway Society - toy trains on 8" gauge (the type you can sit on, not in) going round a little woodland track. Only opens once a month, first sunday, and is in very obscure location, so if your friends are interested, CAT me and I'll let them know where.
Depending on kids English comprehension there are several cinemas(Arts is particularly good and does slightly more 'offbeat' kids films, maybe even french subtitles, who knows?)
Events during the summer that they might find entertaining: mid July there is a weekend festival on Parker's Piece, with free (rock) music, performance art, a 'tea dance' marquee and a pseudo french market. Also a children's theatre marquee at the beginning of August: more details on both here . End of July is town bumps: fine opportunity to take a walk up the river and watch large squads of boaties trying to knock lumps out of each other.
There's normally a circus sometime in August too, but I haven't seen any adverts for it.
For the grown ups I'd also recommend the Shakespeare in the Colleges season which goes on all summer in the college gardens. Normally a fun evening out.
Outside Cambridge Shepreth Zoo is apparently better than Linton, and I like Audley End house too (has a country fair at the end of July I think). If any of them are horsey the National Stud/horseracing museum in Newmarket are worth a trip.
Once you've done all these parochial pleasures you should also bear in mind that a trip into London is a 50 minute train journey (plus tube on arrival, obviously) so there's a lot more scope down there.
Enough. Until the Tourist office starts to pay me to bore you I'd better leave off now. Any questions, just CAT.