Mosaic, fwiw, my thoughts are there wasn't much more to it than venal opportunism from the Romans at that point.
Romans also ate dogs occasionally themselves I believe, so I don't think that, as an exchange mechanism, was any particular humiliation in itself. On a frontier, they're a quickly reproduced mobile meat source with a handy working function, pre-catering stage.
There'd already been a good weed out of the refugees allowed to cross the Danube in the first place, with only the fit, healthy and the nobility involved. So trading one (starving) Goth child for a dog to feed their family gets top quality goods at a relatively knock down price in very much a buyer's market.
I've also seen another account - and I can't for the life of me remember where, or the source, sorry - that instead says the children were "taken," to be educated as Romans in Thrace, and it was the Thracian hosts who reneged on the deal and sold them on for dogs.
Either way, I think the same circumstances probably applied.