30X28 is pretty small if you are going to have a dedicated section for the kiddos. I'd look into Attic trusses or Gambrel roof trusses with living space to give you extra space. As an added bonus when they get older, you'll have a ton of storage space to utilize.
+1 regarding the use of attic trusses to gain inexpensive second-floor space! I used attic trusses on my shop build (34 x 54) and ended up with a second-floor room 12 ft wide x 54 ft long x 8 ft ceiling height. You can see some early construction pics of this space at Post #49 on my build thread:
http://www.garagejournal.com/forum/showthread.php?t=98038&page=3
I didn't finish the space and use it only for storage of junk. Allowable floor loading is about 40 lb/sqft and, if finished, would make a great play room for kids. The attic trusses are somewhat more expensive than simple trusses, but not all that much more and makes for really inexpensive floor area. Add a dormer or two and you could have a really cute building!
You will need to address all the issues of insulation, HVAC, etc. and will need to assure you have adequate fire retarding ceiling in lower garage space, etc. Also, if you make the second floor a living space, you should make sure that there is egress provided from the space that does NOT pass through the garage (this is a building code in my area). Basically, the stairs to the second floor can be built within the outer wall structure, but there must be a door directly outside rather than only out via the garage as mine is built (can not make my second floor living space without adding an additional exterior exit door from the stairway intermediate landing).
Anyway, check out attic trusses as they may be the solution to the extra space you need.