I think it's important that primary school children go occasionally (during Advent and Lent for example) as it means that when they get older and things get serious it is familiar and not frightening. If they get themselves into trouble and tie themselves in knots when they are teenagers they then have the possibility of receiving the sacrament, knowing it is totally safe and 100% confidential. Encouraging our children to go is like giving them a gift that they can fully open if they wish when they are older and more in need of it.
As to what a 7 year old confesses, you can get simple examinations of conscience for primary school children to read through and think about before they go in (have they been mean to a friend, have they refused to help at home, etc. etc.).
If children don't go, it becomes a problem, because they have their first confession before their first communion at 7, and by the time they are old enough to make an independent decision to go, they have completely forgotten how it is done, and it becomes a frightening thing - which it isn't.
I didn't go as a child, and initially struggled as an adult, it took a long time to overcome my fear of it and properly rediscover it. When dd1 was 7 or 8 I posted on here under an earlier posting name about whether I should take her. I did. She's at secondary now, and happy and confident about going. She's comfortable talking to the priest and I hope that she will always be confident in going for the sacrament because it is a wonderful gift.