Children are smarter than we think
If adults were able to learn at the same rate as a child we would all be much smarter than Einstein, Tesla & Feynman put together. Learning slows down as we age not because we want it to but growing brains are making many more new connections. It no doubt has a lot to do with survival.
Watch them learn
Think about how quickly a child learns to speak a language, walk, read, socialise and about a thousand other things all at once. Why then would the guitar be so impossible? It's not. Those who promote guitar is only suitable for 6yo and over just don't know how. A two year is more than ready to learn guitar but we just need to understand as adults how to do it.
What parents can do
It is not so much about the child but the adults (parents, teachers etc.) around them that make the difference. Firstly parents need to understand that just because their 2 year old is just banging the guitar at first it does not mean its a waste of time. Children need to explore the guitar and learn to associate it with playing. If a parent picks up a guitar each day your child will probably do the same and this is the first step.
Find the right teacher
All teachers can teach young children but many simply lack confidence and experience. Some even believe its impossible so as a parent don't be put off if you have trouble finding a teacher at first. Try asking around in your area until you find the right teacher. In the meantime you as a parent can take up the guitar and this will encourage your child to also learn.