Children at the park

I posted this on the fm forums but didn't get much of a response so I'm looking to you guys for some more critiques.

I decided it was too nice a day to waste studying for organic chemistry so I went to the park with my camera. I have shied a bit away from taking candid pictures of strangers but the children playing were just too great of subjects to pass up. I was a bit nervous at first but, me being younger, I didn't get too many looks.

Anyways, here are a few of my favorites.

All pictures taken with a Canon 350D and a 55-250mm IS. ISO was 400 because I had been chasing a squirrel around and needed a high shutter speed.

1.
179mm ISO 400 1/1000sec f/5.6

I'm going to have to say #3 is absolutely adorable, but in the others there isn't that same joy of a child at play.

I do like #5 as well; I can imagine that he was just told he couldn't have any ice cream, and now he's sulking playing with the stones.

i don't know if it's just me, but the focus seems to be a little off on some of them, like #1 and #3 seem soft even though the children fill up a large portion of the frame. In #2 it seems the focus is on the front wheel and #4 it seems the focus is on the back wheel. #5 the focus looks good. the composition on all of them is OK, maybe a little too centered for my tastes.