Five is too little to tackle guitar. His hands are too small and the connective tissue is too soft and immature to take on an adult's job. He also has his intellectual plate full by needing to learn to read, etc. Quit pushing the kids.
There was an article in our Catholic newspaper recently. The head of instruction said that children from well-off homes commonly have high levels of stress chemicals in their blood streams because their parents are pushing them so much. The children get the message that unless they are at the top of the heap, their parents won't love them.
I have a friend who is an excellent piano teacher. She doesn't want to start lessons until the child has mastered reading - about age 7 at the earliest.