Hi Stephine,
I watch children also, I used to nanny for one family, my daughter was 3months old, the boy I was watching was 1.5yrs old, well, he started going to hit my daughter when I would tell him No. Luckily, his mom did not like it either and we nipped that in the bud. For your situation however, I would try a sticker chart, I know you said he doesn't understand, but I would try anyway, except I would also do it for his sister, or any older kids you may have, they get a sticker before they leave if they have had no time outs that day, then at the end of the week, or their last day of the week, they get to pick out of the good behavior basket ( things safe for them from the dollar store, or fav. snacks to take home) after a few weeks of him not getting to pick, and his sister getting to pick, he might get it. Also, I think he likes the attention, even though he is not getting to do the craft,snack, or play, he is still getting attention from you, and from the other kids watching him be punished. I hope this helps, let us know how it goes.