In our house we have a 4 1/2 year old, a 2 year old and a ball pit.
When the 4 1/2 year old began being able to climb out of the playpen (about 2 years ago) we decided not to get rid of the playpen, but to repurpose it into a convenient place to put all of the various size balls that she had amassed. Then, one day, while browsing on Amazon, I noticed the many different – and expensive – kids of “Ball Pits”, then I noticed replacement balls. BINGO! The playpen became our ball pit.
As a matter of fact, this isn’t our daughters original playpen/ball pit, the first one got so much use that we had to replace it. Oh, and the ball pit balls are available at Amazon, K-Mart, and Wal-Mart, and I’m sure most of those aren’t original either!