Just answer the interview questions. Start by saying "someone else" as the relationship. If you don't get the dependent, it's because the child doesn't qualify. An unrelated child living with you can't be your dependent unless all of the following are true:
1. the child lived as part of your household for the whole year
2. you paid more than half the child's living expenses
3. the child earned less than $4050 of taxable income
4. the mother is not a taxpayer for 2016.
The mother is a taxpayer if she has income more than $10,350, or if she had less income but files a tax return in order to claim any credit or benefit other than a simple refund of withholding. If the mother is a taxpayer in 2016, she must claim the child and you can't claim her, even if the mother is willing to allow it.