I suspect you are trying to do too much with kids who are still really young. Simplify, simplify, simplify. Children this young only need an hour or less of "schooling" at this age. Anything more is probably just too much. They developmentally do not have the attention span of older children. And developmentally, children this young learn through play.
You don't say what type of curriculum your "school" uses, but you might want to start tweeking there. Avoid workbooks, and "desk" time. Alot of homeschool beginners make the mistake of trying to recreate a bricks and mortar school at home, and this seldom works for anyone.
You have to think outside the box with homeschooling...otherwise you ought to just send your children to a traditional school.
Any chance you could join a co-op in your area? You might greatly benefit from meeting up with actual homeschooling families (particularly very seasoned families) and glean some support, guidance, knowledge and ideas from them. While no two homeschools are run the same, you can learn greatly from the mistakes and wisdom of others who have gone before you.
Most homeschooling preschoolers will use some sort of pre-reading/phonics curriculum.
These are popular choices:
www.hop.com
www.http://www.amazon.com/Teach-Your-Child-Read-Lessons/d...
For math, alot of people use Saxon Math, Math U See, Singapore Math or The Right Start Math curriculums. They all have different approaches, but all utilize manipulatives and visuals to drive interest and help with abstract concepts. Just do a search on amazon or go to their web pages directly by googling to get more details on each one.
Include some type of art, handwriting, and a review of basic concepts like colors, shapes, etc. That's more than enough for the formal curriculum for preschool. The rest of his learning should be through play, visits to museums, zoos, and other educational activities.
You also might benefit from reading books on designing curriculums for homeschoolers.
A widely used resource by many homeschoolers is a book titled "The well trained mind" by Susan Wise Bauer. See:
http://www.amazon.com/Well-Trained-Mind-Guide-Classical-E...
There is also an accompanying website with resources at:
http://www.welltrainedmind.com/
Many homeschoolers also use the Core Knowledge, What your "N-th" grader should know" series. See:
http://www.amazon.com/s/ref=nb_sb_ss_i_2_21?url=search-al...
Lastly, www.hslda.org is a great resource too. Click on the button that says "You can homeschool" and read everything in this section. Especially the getting started section. It addresses the challenges parents experience with preschool/early learners. The resources are priceless. I highly recommend the section on discovering your child's learning style. This will help you determine which type of curriculum your child will respond to, based on their learning style.
Some kids are visual, others auditory, some are kinesthetic. Finding out which type of kid you have will make a big difference in the type of resources and curriculum you will ultimately invest in. Perhaps the tools you are using just aren't clicking with your child's learning style.