Would be fun kid hideout space! Awesome dog kennel under the stairs design idea. If you want an indoor dog house, utilizing the space under the stairs for a cozy, attractive and practical space for dogs is a good idea! I love this design.
Instead of doing complex criss cross wood design, just do plain planks, stain light sandy colour and pain on a beach scene or incorporate a printed pic. Give to mom as doggie door (for keep dogs off stairs).