Looking on the internet deeply has found these results:

in playground is the most popular phrase on the web.

at playground 1423,000 results on the web	in playground 1449,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Finding Leander Anton... disappeared in a playground in Silverhjd, enters its seventh day. You're too old to be hanging out in the playground. chemical toilets, waste water, hospital waste, soil or other substrates (in playgrounds). The big girls in the playground said it. Kids get it in playgrounds and at schools and E.R. waiting rooms. This mobile phone footage was filmed by a child in the playground. He had a little accident in a playground. They don't worry about abandoned fridge in playground.

at playground

1423,000 results on the web

in playground

1449,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Finding Leander Anton... disappeared in a playground in Silverhjd, enters its seventh day.
You're too old to be hanging out in the playground.
chemical toilets, waste water, hospital waste, soil or other substrates (in playgrounds).
The big girls in the playground said it.
Kids get it in playgrounds and at schools and E.R. waiting rooms.
This mobile phone footage was filmed by a child in the playground.
He had a little accident in a playground.
They don't worry about abandoned fridge in playground.