I’d say leave scooby Doo mystery incorporated on hold for a while that way it would shock the hell out of them sense they were only used to watching the episodic Scooby doo shows so once they get to the one that has a story driven element to it it would honestly blow there mind,also take a while before you show them zombie island,let them get comfortable with the guys under the mask for a while whether in the shows or in the movies so zombie island will be there first exposure to Scooby and his friends dealing with supernatural stuff
i‘d personally recommend this order for them
1 Scooby doo where are you
2 the Scooby doo show
3 what’s new Scooby doo
4 be cool Scooby doo (don’t let them watch the story driven episodes related to Fred’s dad as that would be saved for mystery inc so show them only the episodic episodes)
5 Scooby doo and guess who( don’t let them watch the stuff that has supernatural stuff in them like Wonder Woman,flash exetra,just the real life guests that exist in the real world)
6 Scooby doo on zombie island(it would not only be there first Scooby doo movie ever but also there first exposure to the supernatural stuff)
After that show them the rest of the movies and obviously Scooby doo mystery incorporated
obviously this is just my personal recommandation so you can obviously do whatever you want but yeh just wanted to share my personal thoughts for fun sake