How old are the children first of all?
Kitchen: Never keep chemicals under the counters. Put them in a cabinet or on a shelf in the laundry room.
Only put dishes in cabinets you dont mind your children playing with(pots,pans, plastic containers.)
Bathroom: Keep the door shut. If they can open the door make sure all things that are at their level is up higher that you dont want them getting into. Can even put their tub toys in the lower drawer or cabinet.
Bedrooms: Dont use nightstands or something they can knock over when playing. Dont put breakable things on the dressers.
Yardsales are great places to find little things to fix up the house and not always alot of money. Sometimes you may find some little shelves that can be hung on the walls out of reach of children. Rubber bands help keep cabinet doors shut if you stretch them across the handles.