Are there auditions for the main parts?
In the school I work in, all children who are interested sign up for auditions - and these are singing, speaking, dancing and acting auditions because all those skills are needed.
Then the parts are allocated. there are a finite number of children who are really good at all ther things involved - and if there is a straight choice between 'the child who also gets picked for academic / sports competitions' and 'the one for whom the play is the only thing they excel in', then the latter will get the part - but only once overall ability in the part has been considered.
However, because it is an open audition process, although the casting is sometimes contentious, there is an evidence base to back it up.
If parts are allocated without auditions, that is a different matter. perhaps a better way to go if that is the case is to ask whether auditions could be held each year, so that the process was clear and more transparent for all.
If your DD was auditioned, and was allocated a singing part, that will almost certainly because other children were better than her on the day for the acting parts. There ARE many occasions in school when less able children are given tasks that others might be better at - but a high profile end of year play might not be that occasion!