You keep saying 'drama school'. If they are both keen on MT then you shouldn't be looking at drama schools. Drama schools are for actors.
If they want to be in the West End in musicals, then they need to be trained in ballet, jazz, tap and singing. Possibly commercial dance as well. That's the only way they will get into MT colleges at 18.
Local theatre schools that do loads of all-singing-all-dancing spectaculars tend to often concentrate far more in rehearsals and in coaching 'elite' teams than they do on proper training in classes for a solid grounding in good technique.
What does the school mean by 'musical theatre classes'? What actual technical training is provided in those classes and in which disciplines? What are the professional qualifications and experience of the teachers? Where did they train?
Take a long hard look at the student successes of your dc's school. Do they get a lot of students into top full-time performing arts training? If so, where? And what happens to them after that? Do they go off and get jobs in the West End, or as backing singers or dancers on tour with big pop stars, or do they get contracts on Princess Cruises, or do they end up back at the school they started from, and end up getting basic teaching qualifications so they can teach the next generation of wannabe kids?
Sorry to be brutal, but this is a hard industry to break into, and for 95% of even the most enthusiastic and ambitious, it is only ever going to be a hobby.
By the way, lots of kids give up ballet because it starts to get a bit difficult, not because they don't need it. Many dance schools refuse to let kids do modern dance or jazz classes unless they do ballet as well. It has to be ballet + other classes. It really is that important for technique in the other disciplines to have good ballet technique.