I started praying with my oldest, I don’t know, around 2? Maybe younger. But from there, for years we had a family Rosary most nights, so from the time kids were born. As infants, of course, I or one of the older kids would hold them. We often lit candles and had a large statue of Mary and I think they came to associate it with a very peaceful time.
As they saw the older kids take turns leading decades of the Rosary, I think it became a thing they wanted to do…like the big kids.
I would also suggest seeing if you can find a children’s book, something pretty simple, with pictures and short stories of each of the mysteries of the Rosary.
Sitting still is really an individual thing. I still hate sitting still. Some of my kids find it easier to do. I have boys with ADHD/possibly on the autism spectrum. I let them hold a cat or pet the dog while we pray. If their hands are doing something, I think it helps them ‘sit.’