I don’t know what your budget is but what I have for my boys is a large ferret playpen with an extension. I purchased mine from Fosters & Smith. It has the little mat to go beneath in case of accidents which is really very nice and it’s cute. I don’t know the exact size but I can tell you that it takes up most of my living room. I can't say for certain but you might be able to find a more reasonably priced dog playpen but these will typically have wider bar spacing. Even the ferret one I have wouldn't work for baby rats. When you’re looking around, just be sure that you pick one with narrow enough bar spacing to ensure escapes don’t happen! And, because rats can jump awfully high, make sure you’re getting something sufficiently tall!
A large, tall playpen can get pricey though – especially considering how much we already typically pay for good cages! I the youtuber Shadow the Rat uses a homemade playpen made out of cardboard boxes taped together. She has a tutorial up titled "How To Make a DIY Cardboard Playpen For Rats (Foldable)." As long as you’re supervising them and you're able to keep track of all of them, I think that this would be perfectly fine. They aren’t going to chew through in an instant and if they’re anything like my boys, they’ll be more interested in playing with their human than trying to escape anyway. Just to be safe, I'd make sure to ensure they're in a safe, closed-off room though.
