I got a Rosary CD from the Mary Foundation (through http://www.catholicity.com) to listen to in the car. Now my 4 yr old asks to listen to it Alll the time! She likes reciting along with it. She also has her own plastic rosary, but really gets a kick when I let her use mine.
We started with just one decade and that’s all I say with her right now. That’s just where her attention span is. Older children can probably do more.
Just be consistent with it, let them see you reciting it…it’ll “click” with them.