Your child may have different abilities when it comes to playing in the playground. The question is, are they not wanting to play with other children in the playground? Is it something involving them no wanting to be with their peers, or is it something that they're unable to do. Are they unable to climb? Are they unable to go down a slide like their peers, like their other friends are? And perhaps that's why they're shying away, because they're having difficulties.
Either situation could be a cause for concern. If they're not engaging with their peers and they should be at that age, that's a problem. If they're three, four, or five years old and they're not really wanting to play with their friends, that could be a cause for concern. Perhaps at a younger age, two and three, you have to model and show them how to play and be kind of the tool between the two children. But at an older age they should be seeking to play with their friends, and if they're not in the playground, that could be a cause for concern.
It could be that your child is unable to do things proficiently like climbing the stairs on the slide, and that could be a cause for concern, because that's limiting their ability to keep up with their peers. In that instance, you might want to share your concerns with your pediatrician. Because perhaps your child's gross motor skills, which are those big muscles, those skills that allow you to run and climb and walk, catch and throw a ball, perhaps there's a delay in that area, and perhaps that requires a physical therapy evaluation to further assess your child's development in that area.
So again, observe your child. Take note of what's going on. Why aren't they playing in the playground? Discuss with your child's pediatrician, and ask them whether or not you should pursue further evaluations.