One would think that Karen and Melody (and Gloria I guess) would go out of their way to NOT tattle on Jimmy, since they like him so much. But, they are "little kids".. that's what they do. I think Karen's being on 4 teams (is that too much?) makes her seem a little older than she is supposed to be. Melody as well, who wonders why she is so attracted to bad boys. Makes them seem like they'd be around... 12 or 13 years old? I dunno.... But many, many 12 year old girls are as tall or taller than alot of 15 year old boys (girls get their growth spurt earlier, etc etc).
Sheldon and Pedro both seem closer to 10 years old than 13, while Karen and Melody seem closer to 13 than 10. My guess is they are all "supposed" to be about 10/11.