Vegetable arches can be used to create a special hiding place for your children and grow yummy vegetables at the same time! You just need some flexible PVC type tubing, some netting, vine growing plants such as cucumbers, squash and beans and a place to plant those plants. Of course you can always use vine growing flowers as well.
Interweave the netting through the piping and then place the piping either in the ground or planter boxes. If you need extra stability you can use some straight sticks or rebars placed inside the tubing and then stuck in the dirt.
As a learning extension you can have your children research the benefits of using arches in architecture. There are many cultures across time that have used arches so you can have them chose a culture to research as well. Heck, depending on the age of your child, you could have them research the business origins of the other well known golden arches.