There is no one instrument that is best to learn for children, and often the best one can be the one that they are most interested in!
If your child does not have a preference, and you are unsure we suggest keyboard as an excellent beginner instrument. This is because the children can see the notes, as they are visually laid out in front, and this also helps with learning their position on the music staff.
Playing the piano/keyboard is also an excellent cross lateral activity as it requires both hands, and when you add listening and reading music to that the brain is getting an awesome work out!
We also recommend keyboard because little fingers can play the keyboard more easily than the piano and most woodwind or stringed instruments.