This movie is a follow-up to the original film Dragonheart not a sequel. That's why it's called Dragonheart: A New Beginning.
You can watch this movie without ever having seen the original, the story line of this movie doesn't have anything to do with a continuation of the original story line.
According to the Extras making of the movie that's included in the US DVD version, this movie was to be a Dragonheart for kids and families.
And that's exactly what it is, and does a GREAT job of being just exactly that.
It's rated PG, whereas the original was rated PG-13.