My 4-year-old has an obsession with the little girl across the street.

The neighbor is 8, takes the bus to school, gets to play by herself outside. You get the picture.

Anytime my daughter sees the neighbor girl outside, she screams frantically: "There's my FRIEND! I see her! I want to play with her!"

Once the girl knocked on our door to see if Addie wanted to play. Addie got so excited I thought she was going to scare the child with her enthusiasm.

I have let them play together several times. It always ends in tragedy when I tell Addie it's time to leave. The tears are endless, and the 8-year-old just stares at such extreme behavior.

But it got me wondering: Is it OK to let my 4-year-old play with an 8-year-old? And why would an 8-year-old want to play with a 4-year-old?

Normally, when I take Addie to play with other kids her age, I go with her. But it's different for 8-year-olds. Moms don't have to go along on playdates.

I worry that this older girl will break her heart or be mean to her, or introduce her to concepts and shows like "High School Musical" that I don't want her exposed to quite yet. Then I think what is the harm? Addie loves to play with this little girl.

Gary Direnfeld, a social worker in Canada and expert in child development, said children of different ages playing together happens frequently in rural communities. And, of course, preschoolers often have much older siblings.

So it's not that having such an age difference in a playmate is wrong.

But, a 4-year-old and an 8-year-old are at "different places cognitively, emotionally, morally. This can lead to confusion, more so in the mind of a 4-year-old."

I have a good friend whose 4-year-old has played all day with a 10-year-old neighbor girl. My friend even fed the girl lunch and took her to a movie with them. She, too, wondered why a 10-year-old wanted to hang out with a 4-year-old, but her son was so happy.

Direnfeld said parents have to listen to their guts.

A large age gap between children, such as a 4-year-old and a 12-year-old, can make the interest from the older child seem a bit odd. There is nothing inherently wrong with it, he said, but parents have to wonder why the older child doesn't play with children their own age. One reason could be that the child may enjoy playing a mother or father role, he said.

"We want to know who these other kids are, where they're coming from," he says.

Direnfeld said my concern about what Addie is exposed to is legitimate.

If there is more than three years in age difference between the playmates, Direnfeld said to watch out for the power differential and the possible abuse of that by the older child.

Bottom line: It's OK for kids of different ages to play together. Just follow your instinct. Make sure they leave bedroom doors open, check on them often and treat the playtime as if the kids were swimming in a pool.

So, much to Addie's delight, I'm going to continue to let her play with the neighbor girl, but within reason. I'm sure there will be many more tears when she has to leave her friend.

But right now she's just delighted her friend gave her some hand-me-down Barbie cars. Apparently they are much better than the one I gave her.