It’s never a good idea to photograph Mayan people without their permission, as they consider it highly offensive and it intrudes upon their spiritual beliefs. The old photographers’ rule contending that it’s easier to apologize (for taking a candid photo) than to ask permission doesn’t really apply in Guatemala.
This is especially true concerning photographs of children, and you should be careful not to show them undue interest and attention, as persistent rumors of foreigners involved in child-snatching of Guatemalan children for organ transplant abroad have led to mob incidents on at least two occasions, with two people killed and one seriously injured. (The last incident was in 2000 in the village of Todos Santos.) In both cases, the foreigners were trying to photograph a child. This scenario is most plausible in the highlands, though not exclusively so.
It can be understandably difficult at times to refrain from taking photographs because Mayan children (and Mayan people in general) are especially photogenic and can provide some wonderful opportunities for portraiture or candid shots. On the up side, the situation forces you to interact with the locals and get to know them. You’ll soon find that many are willing to let you photograph them (often for the promise of sending them a photo) and your photographs will be better because of the rapport you’ve established with the subject.