where they understand that correctly exercised it is not painful
The key here is the correctly exercised bit and like so many things it's about getting their technique right. We've started using tackle bags (the cylindrical bags not the shields) this year and it's made a huge difference. Kids will happily hit tackle bags all day long, which means that as a coach you can keep correcting them until they get the approach, body position, hit and follow through right.
We vary things around: i)start with the bag upright and static, coach holds 1 tackles, 1 holds 2 tackles etc, ii)have the coach tow the bag behind him and the kids practise tackles on the move (side-on and behind). As a bit of fun at the end we'll let them do diving tackles onto the bag.
It's one of the most popular modules we do with them. Their confidence is improved because they can make tackles at match intensity without hurting themselves and most importantly we, as coaches, can really work on getting the technique right.