Probably going to be hated for this... But I really don't think it's that big of a deal. The whole point is that it's a treat that a different kid wins each week - so each kid usually gets one voucher a season. Not everything at McDonalds is total crap anymore either which is good. Definitely not something to encourage your child to think is the norm but as a once in a blue moon thing I don't see a problem with it.
As for whether it's appropriate in kids sports for companies like McDonalds to sponsor - again I don't see a big issue. They put money & funding into sports which help keep them going, and kids are exposed to marketing everywhere now anyway - at least this way they get something back and it's encouraging a healthy activity in sports.