I mostly spray dancers and have sprayed my own two for 5+ years for both national finals and regional competitions, so I hope I can help
Speaking from the perspective of Latin/Ballroom/Disco/Freestyle comps etc - they generally start with doors open from about 8.30 to 9am depending on the comp. The first rounds for Under 16's tend to start at around 9 - 9.30am - again, depending on the competitions. Adults generally tend to start later, except at large, national finals. Many people travel in excess of 1 - 2 hours to get to a comp, so with that in mind, you'll have to tan the day before.
As an aside, I was up at 4am last week for a comp :cry:
The only "instant" spray tans I know of, are the Sienna X one (I think it's called Tonight's the Night or similar) and the Wow Factor Organics Illusion Rapid tan, which I'm about to try tonight on my eldest. I don't know whether either of these will be dark enough for Latin dancing.
Jantana do a very good instant tan which comes in lotion form, called "Competition Tan", if you wanted to consider a manual tanning - rather you than me, on wriggly children lol! It's amazing and you don't have to worry about applying it carefully - just rub it in and away you go! Brilliant and instant. We love it for those little emergencies on the day.
There are other instant tans on the market, aimed at dancers but one in particular goes green after 2 hours - bleurgh! Not attractive and it was a very expensive mistake for us.
Please bear in mind that not all dancers like to look that dark and you'll get some who will be happier with say, a 10, 12%. It'll be a shock for them and may knock their confidence on the day. So be prepared to take along some lower DHA's
Also some of the children will be gong to school on the Monday and from 11 years of experience, I know they can suffer really badly at the hands of their classmates from the "tango" jibes.
I sprayed my youngest with Wow Factor's Organics Allure 10% and that was absolutely perfect for anyone under 12 - or even for those who prefer to look tanned but not over cooked lol!
What age ranges are you doing btw?
HTH and I'll be happy to help more if you need a dance perspective lol!
Oops forgot to say that if you're doing young ones under 12, you'll need eyes in the back of your head to try to stop them from touching their skin after you've tanned them...you may need some parents to help you with that one, or some hairdryers ;-)