You mention you work graveyard hours... How much sleep do you get on a weekday? And how does your sleep pattern change during the weekend (as in, do you stay on the same schedule all the time or does your sleep hours flip when your not working)? The reason I ask is, you might live in a state of constant sleep deprivation or decreased REM cycle sleep, both of which could dramatically decrease your cognitive processes (math equations, reading comprehension, etc.) and memory retention.
Sleep question aside... Here's a little game you can play (and your kids can play with you, race against each other, etc.)... especially helpful when waiting at doctor's office, long car trips, etc.:
Get a pad of paper and some index cards. Start with 60 cards. Write the number 1 on one card, then 2 on the next, then 3, etc. up to the number 20. Ten on the next 20 cards, number them again. Do it again for the 3rd set of 20 cards. Shuffle them up. Shuffle really good...
Lay out 5 cards... so you have 5 numbers. Lay out your sixth card. Practice figuring out a simple math equation (using combinations of adding, subtracting, multiplication, division, throw in taking the root of a number or squaring if you want) so that you use the only the numbers on the 5 cards (and all of those numbers) to equal the number of the 6th card.
For example, let's say you draw the following 5 numbers:
14, 2, 7, 11, and 9; Your 6th card is the number 18.
Can you use the numbers together to equal 18... hmmm, let's see...
14+11= 25
9+7=16
25-16 = 9, times the 2 = 18
Use the pad of paper to work out the math... Over time, see how fast you can solve it, or see if you can do it in your head. If you get to a set of numbers you can't solve, write down the numbers on the pad as something to work on "later"... You can always add another set of 20 cards or use numbers over 20, as needed.