It depends on her age when it counts, not on her age at filing. The Child Status Protection Act allows certain processing time to be subtracted from a derivative's age, so her eligibility should be calculated on the actual dates rather than assumed either way.
Calculate the age; do not estimate it
A derivative child must be unmarried and under twenty-one, and the risk in an investment case is that adjudication and visa availability take longer than the child has. The Child Status Protection Act provides a calculation that can hold a child's age below twenty-one by subtracting the time a petition was pending, subject to the requirement that she seek to acquire the status within a set period after a visa becomes available. Because that calculation depends on the filing date, the approval date and when a visa number becomes available for the category and country of chargeability, it produces a real answer only when the dates are known.
Two practical steps follow. File as early as the capital and the project allow, since every month of delay works against her. And review the visa bulletin for the category, so that the family knows whether the constraint is adjudication time or visa availability.
If the calculation runs out, plan an independent route for her before rather than after.