@Playdoughcaterpillar, my DS did a lot of professional MT (including WE) when he was younger and I am familiar with a number of holiday training courses. I think whether a West End Stage course is worth the exorbitant fees or not depends on what your child is looking to get out of it.
If your child's main aim is to spend a week getting some high-quality training amongst like-minded kids, something like Spirit Young Performers, Stagebox or MX Masterclass would be much better value. Staff on those courses are often working MT professionals.
If your child is just excited about the experience of performing in a WE theatre, then West End Stage might be worth considering if you can afford it.
However, if your child is keen on the couse because they hope they'll be spotted by agents or casting directors, it's definitely not worth the money. Despite the hype on the West End Stage website, casting directors very rarely use these sorts of courses for serious scouting, and a child is more likely to get noticed by an agent and offered rep from a summer course run by a performing arts school with an affiliated agency. Stagebox would fit the bill there, or if you're anywhere near North London, I can recommend New London Performing Arts Centre in Muswell Hill.