Halloween // Ideas for family of 4 but can be just 3. Preferably diyable

By Halloween will have a 6 month old girl and a 3 year old boy. Dad may be working so costumes need to work with or without him.

I'm cheap so while I'm willing to buy store costumes for the kids I'd rather not for the parents (my pooh costume last year was a cheap yellow dress and red crop top vs buying a $70+ licensed costume

Any ideas?