I can relate to what you are saying about the fact that you definitely don't want to neglect your son. You should not neglect him but really, at his age, you should be able to accomplish his actual 'school work' in less than one hour. Are you teaching him to read? That does take some time and it is very important but even that should not be for too long each day. Some ideas for other things to fill his time that are educational but also fun............
watching educational dvds *you can get tons of these from your local public library.........on all kinds of subjects.......especially science-related ones
playing with legos
listening to books on cd/tape (read-alongs; where he listens but follows in the book)
playing educational games on the computer
I think he needs time this year *and even next year* to just PLAY because he is so young. So during that time, you can focus on helping the girls to get them caught up. By the time they are caught up, then you will be able to give him more attention. (by then, he will be in 2nd grade)
My advice for working with the girls would be to focus on the 3 R's -- really stick to the basics. Make sure they can READ - REALLY READ. Once you have accomplished that, they can learn SO much independently.
For math, I highly recommend Teaching Textbooks. It is a wonderful curriculum. It comes with solutions cds, so your child will always be able to see WHY they missed a question and they will be able to fully understand. The important thing for the girls would be to buy the right level, no matter what it is. The Teaching Textbooks site has placement tests on it. They could take the test so you will know what to buy. It is expensive, but can be passed down to your son. It is worth the money! You could try buying it used also from ebay.
If you choose this program, they will be able to work independently on math, which will be very helpful for you.
For the final "R", I would teach them basic grammar because they are probably behind in that and maybe have them write something each day, even if it is just a journal entry. We like Easy Grammar.
http://rainbowresource.com/prodlist.php ... egory=1750
My favorite place to buy things from is Rainbow Resource. They have a huge selection and low prices.
You could look on their site and find a fun writing prompt book that you think is age appropriate for the girls. You could use it to give them some writing assignments. Then you can help them edit their work, make corrrections, etc.
I would not worry about all the extra stuff right now since they are so behind. (history, science, vocabulary, art, music, foreign language, etc.)
To make it fun, I would suggest going on field trips. Choose some places that would be educational but good for ALL the ages. Also, play some fun board games. Do the girls know their states? We love a game called Name That State. (you can get it at Rainbow Resource) Do they need to brush up on their multiplication facts? We like a card game called Multiplication War. There are also tons of free educational games online that they could play.