It is always a good practice to alternate products with different active ingredients. A great alternative to Demon WP would be something like Suspend SC. Suspend SC is a general insecticide that is labeled for various pests, including roaches, mosquitoes, ticks, fleas, etc. This can be used indoors and outdoors around the perimeter of your home. We would recommend 30 days between applications of different products.
Demon WP Insecticide is safe for children and pets after it has dried.
That is a hard question to answer because both work very well. However, we do sell the Demon WP in our Roach Control Kits and it does work a little faster and provides a punch for quick knock down. Sprays alone usually do not work. You should use this along with a Roach Bait so the roaches have something to take back in the walls and other hiding places that you do can not spray. Our Roach Control Kits have a very high success rate and incorporates sprays and baits.
Mix one packet of Demon WP Insecticide to one gallon of water and make sure it dissolves completely before you begin spraying. This mixture will treat 1,000 sq ft and you will need to re-treat every 14 days (do not treat daily) as needed for flea control. Most folks find they have to do between 2-4 treatments to get rid of the fleas for good along with the use of an IGR such as Archer IGR. You may want to look over our Flea Treatment Guide for more tips and information.