An organist told me that people often complain that singing above C4 (or c5 if female) is too high. Why would people have a problem with singing above C4 C5 if female)?
The problem is that a) adult always sing too low with children when singing baby songs - also primary school teachers and church song books are making compromises to the breast voice.
b) pop songs are always in a range too low, girls are singing in a tenor range and boys are transposing an 8va down. Then the kids have fear and hesitation from singing high.
The point is that they don't use the head resonance, they are fixed in the breast resonance.
I know this problem very well as I had always pupils arguing and complaining like that. They can be helped very easy by singing the tunes first on vowels like i, ü, u, ooh, then opener vowels like ö, ou, oo, e, a. You must work a lot with imagination and imitation animal cries like a horse, a cock, an donkey. Old Mac Donald had a farm is very useful for this.
This teacher tells exactly my experience:
We had a national federal president who said (in political and economical concerns):
the upswing begins in the head
That's exactly the right answer - also for singers!
I actually have to add this Amee Nolte video:
So learn to sing with your head-voice!