I'd say "¿te lo vas a comer?". It sounds much better in my opinion.

Hi, Emma "¿Lo vas a comer?" is totally correct but very inusual in Spanish. Why? With some verbs like "comer", "beber", "tomar", we use a thing called "dativo de interés" (no need to know the name of that thing :)). It means we use the pronoun "me, te, se, nos, os or se" without ANY meaning but only to make insensify the action of the verb. So for a Spanish speaker, "¿Te lo vas a comer?" is so much natural than "¿Lo vas a comer?". Although, "te" means nothing in the sentence. Hope it helped you. Regards.

It has to be ¿vas a comerlo? You only move (lo) when conjugating the verb ^.^