Ideas for handicapping strong players?
Group the players according to skill level. Each of your words are eliminated if anybody in your group or below has a duplicate word. Players in groups higher than yours cannot eliminate your words, but you can eliminate theirs. I think there should be at least two players in the lowest group, but maybe you want to be really kind to the most junior player.

I haven't tested this, it might be too unbalanced. Try experimenting with the group sizes. I suspect fewer groups would be better.