Oh yes, let the kids be creative even if it means a messy house!! lol
Kids love to experiment. They need something to do in their spare time. They'll either draw, make some paper cuttings, create a figure or whatever that is a product of their creative minds.
I know, your house can be really messy just like this but we are sure that kids' brains and creativity is working and not idle. This is their way how to express their feelings, what they think or how they envision of what they want to be in the future.
Who knows they can be an architect, visual artists, designer or engineer one day, right?
My 9-yr old niece is always painting, doodling, making a doll house and making a mess. Her parents most especially her mom will always scold her because of the mess that she has created afterwards. There are also times that she doesn't like to put away her things and that is something that we don't like about her.
Get them off from ipads, computers and other gadgets as much as you can and encourage them to create anything that their minds can allow.