Well my son’s Beinn 20 had Tektro brake levers and when the Beinn 24 I bought needed a new saddle I bought and “Everday” kids saddle from Halfords which was on the same base, had the same rails and the same saddle shape (with different covering).
I imagine that they buy enough components to have their own runs in the same way that some Charge saddles bear a striking resemblance to a number of others.
If you’re looking to build a kids bike I’d recommend the saddle I found for size (http://www.halfords.com/cycling/bike-parts/saddles-seat-posts/xlc-everyday-kids-saddle) whilst the Avid brake levers I’ve just fitted are good for smaller hands.
Cranks are always the tricky thing but there have been a few threads on here over the last few months with options.