Eliza, I'm a current Cub leader and we often share songs etc at campfires with Brownie groups. I've never come across them singing anything age-inappropriate. Its notable that the Brownies tend to sing all the traditional camp-fire songs beautifully (rather better than our boys anyway!).

I think its a fair comment that there is more variation in Brownie Groups who are dependent on 1-2 individual leaders to do everything, whereas Cubs tend to have support groups and committees etc (and very useful they are!). So my guess that what you saw was a one-off specific to that group.