Actually from what you have described, it makes perfect sense that your income tax is nil.  The basic personal amount is $11,327, and the eligible dependent amount is also $11,327.  Between just those two credits, you have $22,654 to help offset taxes...and with your children's disabilities, and getting a credit for the employee portion of your CPP, it makes perfect sense that you would have no income tax owing.  The CPP is calculated as 9.9% of your net income above $3500.  You say your income is $25,000...so I would calculate your CPP owing as being:

($25,000 - $3,500) x 9.9 = $2128.50