Wow I wish my kids had language offered in elementary school!
I teach Second Language Acquisition and I really don't think it matters what language a child learns. The important factor is motivation, which could come from the language itself, the teacher, the method, the opportunities to speak, anything. If you're looking at an hour a week, your child will be learning about the language, but won't get much of a base. If it's a summer course, do the one that's the most fun (right level, everything), but remember that to keep it up, they will need some way to continue exposure. I'd look into finding an ESL conversation partner from a local college who would trade English conversation for their own language.
BTW, I took German when I was young, used it to help the future mother-in-law of a friend who married a German navigate in the US for a week. Then I learned French fluently and I use it all the time because I am motivated to do so.
There are some cool brain studies coming out about the different ways children store L2 compared to adults.
