If you are starting your kids off on this awesome classic series, we suggest you start with this one! It makes the most sense because it doesn’t spoil the surprise about Darth Vader being his father (watch these kids' reactions and you will know why we wouldn’t want you depriving your kid of the full impact of the "I am your father" line either!) and neither does it bore your child with the prequels that don’t have the same level of action.
The special effects are probably not as jaw-dropping as they were to us when we were kids but it’s still an entire universe created from scratch. The great plus point is that unlike us pre-90s kids, they get to enjoy the entire series in unbroken order if they so desire.
We suggest you follow up with the machete order of Part V, II, III and then VI. Trust us, it really does make the most sense!