A minor under the age of 18 can enter the country with either their own passport or on the passport of a parent with whom they are travelling, but if they are not accompanied by both parents, they will need written consent from whichever parent is not with them (or from both if they are on their own).
Travelling with younger kids is not uncommon – most Mexicans dote on children and they often help to break the ice with strangers. The main problem, especially with small children, is their extra vulnerability. They need protecting from the sun, unsafe drinking water, heat and unfamiliar food. Chile peppers in particular may be a problem for kids who are not used to them. Diarrhoea can also be dangerous for younger children: rehydration salts are vital. Ensure that your child is aware of the dangers of rabies and other animal-borne illnesses; keep children away from all animals and consider a rabies shot.
For touring, hiking or walking, child-carrier backpacks are ideal: they can weigh less than 2kg. If the child is small enough, a fold-up buggy is also well worth packing – especially if they will sleep in it while you have a meal or a drink.