I wouldn't do it, esp if the trainer says let the pups do whatever for 15 mins and no interaction? The other owners haven't had their pups long so they don't really "know" them yet. Large breed pups are pretty clumsy too when they play. If they even roll over on her she could get hurt.
Brit is right about socializing with all size dogs but me personally, when there is a dog over 20 lbs I don't even let her get close, I pick her up. And we have a bull terrier at home, they do not interact with eachother, he's just too clumsy and doesn't know if own strength. That said, I do allow Uni to sniff a big dog on the other side of the fence, but never will she play with a big dog. (Like she even wants to play with a small dog, but that's another matter
)
But if you feel uncomfortable then don't do it, better safe than sorry.