Are those ages right? 21, 19, 16 and 13? Are you sure they don't have plans? Not that Disney wouldn't trump most things; but still. I am guessing the 21 and 19 year-old are in college and/or don't have jobs. I have a 16 year-old and she likes to know at least 3 weeks out if she's leaving town. If you're going in August I would go ahead and let them know.
The last time I surprised my oldest she was 11. She had also been to Disney before. Have your children been before or was your honeymoon the only trip? I'd probably do something simple like give them new luggage with Disney shirts in it. And then something that indicates when you are going like some Disney gift cards with Family vacation 2019. Have they been asking to go? If so, something like what I did for my son last year might work. He had been asking to go; so one day I gave him an envelope and said we would do a countdown and inside was a sheet of paper indicating the number of days until we would go. Now I did this on the day we left (he was 8); so on the paper was a big 0 with the words "let's go!" underneath; but you could make an actual countdown chain or calendar and surprise them with it.
I'm trying to figure out a reveal myself. Everyone knows about our upcoming trip; but we're bringing my mother. Trying to think of a creative way to tell her I've paid for the trip and all she'll need is spending money.
I hope you all have a great time and it goes well.