I would definitely wait until a child is at least 5 years-old before enrolling them in any dance form. Kids are less likely to hurt themselves and more likely to understand and enjoy what they are doing/learning at that age.
Finding a good dance instructor depends on what area you live in. The best tip I have is to check out an Arthur Murray or a Fred Astaire. Make sure to take a lesson with an instructor there yourself first; If you like and understand everything they teach, then they might be a good fit for your kids.
Also, while these studios are good places to start, I would actually recommend looking for independent dance studios for long term results. Try joining your Local USA Dance Chapter (or check out their website); They have lists of local instructors and independent studios.