There are two basic routes you can go for the kids bikes, 4-stroke play bikes or two stroke race bikes. The former will have very low end brakes, suspension etc, but very gradual, easy to use power (and not much of it). Two-stroke MX bikes will have much higher quality suspension, brakes etc, make much more power, be lighter weight but a bit more of a handful.
When my family got into riding we went with with older MX bikes (cheep, but still plenty of performance) and that worked out great for my sister and I.
For the 7yo you might be best off starting with a little four stroke. Perhaps a CRF-50 (same basic bike as an XR50 or Z-50) or the yamaha version of the same. Yamaha also makes a PW50 that's really small and easy to ride (auto clutch, single speed trans, shaft drive) but is probably already too small for him (its popular with 4-5yo's).
For the 11yo a KX65 would probably be about perfect. If he's a big big for his age a KX85 would be in order (or any of the other 85cc MX bikes). For a low cost in-between look for an older 80cc MX bike as most were a little smaller than the current 85's physically.
If the 11yo is a really cautious kid, a KLX110 or KLX125 would be good options (or any of the equivlent bikes from other brands - TTR125, DRZ-125 (same as the KLX only yellow) etc).
BTW I started at 11yo on a YZ80:banana
For the Mrs, it really depends on her inseam and riding skill level. My Mrs rides a KX100 and finds it perfect for her as does my sister. The bike is really light (150lbs), makes more power than most anyone needs (25rwhp) and has excelent suspension, brakes etc. Its also super easy to start (unlike most kick start thumpers). The downside is that the engine has to be reved up pretty high to make any real power and when the power does come on it comes on more suddenly than some folks may like. One plus of this is that a timid rider can put around at low rpm and have a very light, mellow bike, then as their skill increases they can start using more revs to get more power.
If the KX100 is a bit small for her a KDX200 is another excelent option. Its a very easy to handle bike that still has lots of performance potential.
On the slow/heavy thumper side, top pics would be a TTR125LE (electric start thumper, physically about the same size as the KX100) and the CRF230 (same as the previous bike, only closer to the size of a KDX200).
Another bike, and possibly the ideal option is a Gasgas pampera. Its somewhere between a KX100 and KDX200 in physical size, weighs 200lbs dry and has a 250cc two stroke trials bike motor. Very slick and especailly suited for chicks or short guys.
Just FYI on the used market, the four stroke play bikes tend to hold their value well, the two stroke race bikes don't so you often will find that the high performance bike will be cheeper than the thumper if your buying used.
Here's a good little write-up on the KX100
http://www.motorcycledaily.com/18aug01kawasakikx100.html
http://www.motorcycledaily.com/01may03kawasaki2002kx100modified.htm
Pampera
http://www.gasgas.com/Pages/2002-s/pampera-250-specs.html