How much they need to earn depends on if they're employed or self-employed:
If they're employed, they need to earn over the lower earnings limit (LEL) in a single tax year to get a year of NI credits. For 2022/23 this was at least £6,396 a year from a single employer. Crucially, though, only weeks where you've earned at least £123 count towards this total.
If the child's parent is unsure whether they're earning enough to qualify for an NI credit through employment, they should check their NI record. Their record will clearly show complete/incomplete years and the breakdown between paid work and/or National Insurance credits.
If the child's parent is self-employed, they need to have profits over the Small Profits Threshold in a single tax year to get a year of NI credits. For 2022/23 this was £6,725. If they do, they'll be credited with Class 2 National Insurance and so the credits earned via child benefit can be safely transferred.
It might be tricky to ask about their profits – so to save any awkward conversations, you can ask them to check their NI record. Their record will clearly show complete/incomplete years and the breakdown between paid work and/or National Insurance credits.