First glancÚ gave me a definite scooby doo vibe. The boy seems a little plain. Not really knowing what you're going for, I hope that helps a little.
I see four 30 year old adults, two of which are kinda short but, I don't see any kids.
The difference between adults and children is all in the proportions. Note in the first example I have NOT drawn 3 separate egg shapes, I've PASTED the same egg shape 3 times. Therefore, the shape of the egg is not the determining factor here. What does determine age is proportion. Place eyes 1/2 way and get a grownup; place brows at 1/2 and get a child. For those aged in-between, adjust halfway point accordingly
On Brunette Girl all I did was drop the eyes (which has the effect of shortening the nose). For the two at right I used the Transform Function to proportionally shrink all facial features as a whole (not the skulls) and dropped the entire face down.
I don't know ages but just to throw out some numbers let's call them 16-14-12 years old. If the gal at left is 16 she should dress like 16, not 30. Swap her outfit for Big Red's (who I assume is Mom by virtue of her height) While it's possible for a 12 year old to have a D cup, if you want to SELL 12 years old, go with no cup.
Feel like the "costumes" could use a bit of personalization, even if it's as simple as a necklace, a watch, a logo on a T-shirt. Just something to make them feel less generic.
On the other hand, you've communicated personalities really well; looking at this pic, I can imagine how they'll act in the story, what choices they'll make, what their dialogue might sound like.