Technically, if you and your lovely woman are not married, you are not supposed to claim her dependents. The reason for this is: they could be treated as "qualifying relatives" for you, not qualifying children. However, in order to be qualifying relatives, the children cannot be qualifying children for another taxpayer. The children are qualifying children of their mother.
Second, if their mother attains a job now, in the beginning of December, she will probably not make enough to receive a worthwhile tax return at tax time.
I will give you an approximate breakdown for what will happen if you get married in 2008, which is probably the most advantageous idea for you two:
Income (approx) - $50,000
Standard Deduction (married filing joint) - $10,900
Exemptions (7 exemptions, you, your wife, 5 dependents) - $24,500
Taxable Income ($50k - $10,900 - $24,500) - $14,600
Tax Liability (approx) - $1,463
For example, if you have had $5,000 federal taxes withheld from your pay, you will receive a refund of $3,537 ($5,000 - $1,463).
Keep in mind, your tax liability is less for a married couple filing joint.