I have a shed full of kids stuff. I use vac bags, but its impossible to sort through the items once the bag is sealed (without needing resealing), so make sure you label well. The medium size ones from kmart are the best I think, because they stack inside a plastic tub and will store then in the shed or attic. Masking tape and black marker pen makes great labels

ETA I got the vac bags because I was worried about mould. The seal on the plastic tubs aren't great (especially when they're jam packed!)