Having a boy 9 (the reason I got back into comics after 20 years) and a girl that is 6, I find the search for the right titles difficult.  I need to be concerned with content first of all.  I love the fact that Marvel rates each title to give me an idea what might be appropriate for them.  And the local comic shop sets aside a section for the kid specific titles.  For the boy, he has a sub to Marvel Adventures Spider-man and Super Hero Squad (which is another topic to itself!) it was difficult to pick out more titles for his pull list.  We've settled on Batman: Brave and the Bold, Invincible Iron-man and Marvel Adventures Super Heroes.  Listening to him read Super Hero Squad and laughing out loud is music to my ears.  
Now, my daughter is a different story.  She's got a sub to Tiny Titans and LOVES that one... can't wait for it to come each month.  On her pull, we have Cars: the Adventures of Tow Mater, Archie and (for now) the Simpsons.  We had the Shrek free comic book day issue and she really like that one.  Unfortunately, it doesn't look like an ongoing series.  We might have to check out Toy Story to replace the Simpsons.  Not that Simpsons is that bad, but... my daughter just doesn't get it.  She says she likes it, but...  Anybody have any thoughts on what would be good for a 6 year old girl that would be an ongoing title?