I've been wondering if any dinosaurs had evolved crop milk. In birds, it's altricial species who benefit the most from the evolutionary advantage, that is the chicks are underdeveloped and need high parental care and grow fast(e.g penguins, flamingos, pigeons).

I think it's believable that some oviraptorids might've had crop milk too. We already know they devoted a lot of time to hatching and raising it's hatchlings.

Maiasaura and other hadrosaurids are a likely contender also, where evolutionarily it makes sense. Their hatchlings were quite underdeveloped and there's evidence of strong parental care. Although they're so large and bulky, it's a bit hard to image them being able to feed mouth to mouth.