We know that the Pandavas were Kshatriyas. Were they consuming non-vegetarian food?
Yes, the Pandavas did eat meat. In this chapter of the Vana Parva of the Mahabharata, Yudishthira tells his brothers that they should leave the Dwaita forest because they've been eating all the deer there, so out of compassion for the deer they move to the Kamyaka woods:
Once on a time, as Yudhishthira lay down at night in the Dwaita woods, some deer, with accents choked in tears, presented themselves before him in his dreams. To them standing with joined hands, their bodies trembling all over that foremost of monarchs said, 'Tell me what ye wish to say. Who are ye? And what do ye desire?' Thus accosted by Kunti's son--the illustrious Pandava, those deer, the remnant of those that had been slaughtered, replied unto him, saying, 'We are, O Bharata, those deer that are still alive after them that had been slaughtered. We shall be exterminated totally. Therefore, do thou change thy residence. O mighty king, all thy brothers are heroes, conversant with weapons; they have thinned the ranks of the rangers of the forest. We few--the remnants,--O mighty-minded one, remain like seed. By thy favour, O king of kings, let us increase.' Seeing these deer, which remained like seed after the rest had been destroyed trembling and afflicted with fear, Yudhishthira the just was greatly affected with grief. And the king, intent on the welfare of all creatures, said unto them, 'So be it. I shall act as ye have said.' Awaking after such a vision, that excellent king, moved by pity towards the deer, thus spake unto his brothers assembled there, 'Those deer that are alive after them that have been slaughtered, accosted me at night, after I had awakened, saying, 'We remain like the cues of our lines. Blest be thou! Do thou have compassion on us.' And they have spoken truly. We ought to feel pity for the dwellers of the forest. We have been feeding on them for a year together and eight months. Let us, therefore, again (repair) to the romantic Kamyakas, that best of forests abounding in wild animals, situated at the head of the desert, near lake Trinavindu. And there let us pleasantly pass the rest of our time.'
In India actually meat was prohibited for a civilized person, here uncivilized means people who does not belong to the society, mostly who resides in forests (tribes). you can find following lines on Wikipedia about it:
The fiend who smears himself with flesh of cattle, with flesh of horses and of human bodies, Who steals the milch-cow's milk away, O Agni,—tear off the heads of such with fiery fury.
The cow gives milk each year, O Man-regarder let not the Yātudhāna ever taste it.
If one would glut him with the biesting, Agni, pierce with thy flame his vitals as he meets thee.
Let the fiends drink the poison of the cattle; may Aditi cast off the evildoers.
May the God Savitar give them up to ruin, and be their share of plants and herbs denied them.
Agni, from days of old thou slayest demons never shall Rākṣasas in fight o’ercome thee.
Burn up the foolish ones, the flesh-devourers let none of them escape thine heavenly arrow.
— Rig Veda (10.87.16-19)
That means no one was allowed to eat meat and its kind of a sin. Surely Mahabharata time period is much before that so i don't think Pandavas ever eaten meat or something, and during there vanvaas they live like Brahmins so there is no question of meat there. Here is the link from wikipedia.
It depends on how you define a "non-vegetarian".
If you define non-vegetarian as a person who eats captive-bred meat (today’s scenario), then answer is No. pAndavas were not non-vegetarian because they did not eat this kind of meat. The reason is because as per mahAbhArata anushasanaparva CXV captive-bred meat is sinful for all dvijas (brahman, kshatriya and vaishya):
He who permits (the slaughter of an animal), he who cuts it up, he who kills it, he who buys or sells (meat), he who cooks it, he who serves it up, and he who eats it, (must all be considered as) the slayers (of the animal).
One should never eat, O chief of Bharata's race, like a Rakshasa, any meat that has been obtained by means not sanctioned by the ordinance.
One source of meat which was sanctioned by ordinance for practicing-kshatriyas was that obtained by hunting of wild animals i.e. venison-game as mentioned in anushasana parva Chapter CXVI of mahAbhArata:
Listen to me as I tell thee what the ordinance is that has been laid down for the Kshatriyas. They do not incur any fault by eating flesh that has been acquired by expenditure of prowess. All deer of the wilderness were dedicated to the deities and the Pitris in days of old, O king, by Agastya. Hence, the hunting of deer is not censured. There can be no hunting without risk of one's own life. There is equality of risk between the slayer and the slain. Either the animal is killed or it kills the hunter. Hence, O Bharata, even royal sages betake themselves to the practice of hunting. By such conduct they do not become stained with sin. Indeed, the practice is not regarded as sinful.
pAndavas consumed venison-game which is fundamentally different from modern day captive-bred meat as seen in mahAbhArata vana parva L :
"Those bulls among men, collecting the produce of the wilderness and hunting the deer with pure arrows, And hunting with arrows Rurus and the black deer and other kinds of clean animals of the wilderness, And the king himself wending towards the east, and Bhima, towards the south, and the twins, towards the west and the north, daily hunted with bow in hand the deer of the forest, for the sake of venison.
The diet of pAndavas thus comprised of (veg + venison/game) as opposed to diet of modern day non-vegetarians which comprises of (veg + captive-bred meat).
pAndavas in today's world would thus be vegetarian due to prohibitions on hunting.