The sukkah is the one building a child is genuinely allowed to decorate however they like, and they know it.
Paper chains look wonderful on day one and are pulp by day three if rain gets in. Decide early which decorations are meant to be temporary and which need to survive — then laminate, or use card and hang under the covered edge.
Children mind a decoration disintegrating far more than adults expect. Saying "this one is just for today" in advance solves it completely.
Everything at child height gets knocked. Run string between the beams and hang from that; it also lets a child add to it all week without an adult and a ladder.
Young children are far happier producing fifteen of the same chain link than one elaborate ornament, and fifteen links actually fill a wall. Repetition is a feature here.
A few days — long enough to be ready, short enough that the excitement is still attached to it.
Practices around what is hung and what is done with it afterwards vary between communities. If it matters in your household, ask your rabbi rather than taking our word for it.
Part of our guide to The Jewish year with children.