Wooden chew toys are commonly found in the small animal section in
pet stores. Also
cardboard boxes are great for them to hide in and chew up. Make sure there are no globs of glue on the boxes, though.
Rats will chew up everything, and may consume some of it in the process. Use common sense; if you wouldn't want them to eat it, don't put it in their cage.
There are lots of great toy ideas online, like at the Dapper Rat site:
Toy Ideas
There is even a special section for baby rats on that page. Good luck.