I gave my kittens toys from about 3 weeks. They liked the small filled pillows and stuffed balls with bells on. They would play with them and then use them as a 'comfort pillow' when they slept.
At just over 4 weeks, they are loving all sorts of toys....especially the cat tree. If you have one or able to get one, I recommend that as they enjoy climbing the sisal climbing posts. I have carefully attached toys (cat teddy bears and balls) to the cat tree and they love playing with the dangling toys (be careful about elastic/string).
At this tender age, its the small sisal toys with bells that I would recommend.