Sifu chose BSL for the kids because it moves around more and emphasizes leg flexibility. From what I've seen of Hung Gar, I think it might be more difficult to teach to children.
Also, I'm the only kids' instructor at the moment (though you met the other instructor yesterday from whom I took over and who still might return). I don't know Hung Gar, so it would be difficult for me to teach it
. I know the basic Arrow Hand set, which might be good to teach to beginners as a little mini-set, but that's all. Ideally, it would be cool to have an option of which style to learn while still having everybody work out together.
Learning both at the same time is usually not adviseable unless one has a solid foundation in one to begin with. This is because certain basic things differ between the two and would impede one's learning at a critical stage (example: foot placement in the bow stance).
Avoid dogmatism in anything. Be open to the possiblity that what you were taught or what you think may, in fact, be completely wrong.