We are also a start at MK and end at MK family. Even do this traveling solo. HS is great and I am a Star Wars fan, so I am not one that thinks this is just a few hours. It is a full day for us as we love SW and shows! However, there is nothing like walking it to MK that first morning and seeing the castle at the end of main street. For your first trip, rope drop and be there when Mickey opens the park from the Castle in the morning it is so magical and your kids are young enough still to really buy in.
HS is a full day with all the offering but it is a lot of shows - I would use HS as a day to help rest tired feet. The shows are longer and you will get a lot more sitting time than you will at MK.
My other thought is though you have lots of Star Wars - lots of that is meet and greet. You have only 4 rides at HS so it feels slow to kids though as an adult I feel rushed to get to my favorite shows. MK is a lot more action as you are going from ride to ride to ride with some shows mixed in. If you start with the action, they might be ready to slow down a bit the next day. Also, not knowing heights they might not be able to ride all of the HS rides as 3 are "thrill" rides.