A spirit offers to give you eternal youth. You won’t age a day, and maybe they can make you whatever age you want, or have you stop aging at a certain point in the future. You won’t be immortal. You will die at some point, but till then you won’t ever age. The price being that the pact becomes null and void once you have your first child. Once the child is born, you’ll resume aging as normal. Would you take this deal? Personally, I would.
Since people are in their prime when they are also fertile this seems pointless.
However this is only the human side of the pact: what’s the rest of the deal?
Maybe they want you to do something that requires you to be young, and they make bearing a child the way to opt out. Imagine how much good one can do if age was never an issue? They could be cop, fire fighter, or soldier, and because they cannot age they can do it for longer. And without familial responsibilities, they can be more devoted. And if they have a child, the spirit lets them out of the pact, presumably looking for someone else.
No maybes. I need to know what the deal is before I make a pact.
Child or no child, opt out or no opt out is irrelevant until I know what the other side of the deal is.
No deal. 
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Like mulberry has said I would want to know the intricacies of any deal before I agree. It will also depend on whether I want children or not. For someone who doesn’t want children this will be a better deal than those who are obsessed with giving birth.
This deal will also depend on what gender you are. If you are a woman you will know when you give birth. If you are a man particularly one who may have separated from the woman giving birth how would you know. What happens if the semen from a masturbatory emission was stolen and used?
Does the pact apply to accidental pregnancies or crimes such as rape resulting in birth? Does an abortion affect the pact or a still birth or a miscarriage?
How about c sections where technically the child isn’t born in the normal way per Macbeth? Without knowing the intricacies of the pact it’s impossible to say whether I would accept the deal