At what stage would access to Xcode on a Mac become necessary?
IMHO, to start with. If you can afford a Mac, that's much better than using iPad.
As long as your daughter does not publish on the appstore (but can still load her apps on devices, her iPhone or iPad), that's for free, no need for a paid developer licence (the Mac is not free unfortunately, but there are some not too expensive machines).
So Mac + Xcode is not necessary, but so much better.
Are there recommended Swift Coding Clubs, education programs
She will have this forum for help. If she follows the good advices from Quinn https://developer.apple.com/forums/thread/706527, she will get incredible support.
first year if she eventually wants to participate in the Swift Student Challenge?
Take care not to put the bar too high. Doing so in the first year may be too challenging and even discouraging. As Quinn said, more important to know what she would really like to develop, why she wants to develop than trying to participate to the challenge on her first year of practice.