I highly recommend you book flights, accommodation and key tours you want to do beforehand. Not only does this save you money but it also means you will have more options available to you.
Airports tend to be a stressful environment for kids if they are waiting around for too long. Most of airports don’t offer much in the way of entertainment, and there’s only so many toys you can pack with you.
It also helps if they can sleep on the plane.
The first day you will all be tired, not just the kids. Don’t rush straight into the itinerary and have an easy day into adventuring.
Bangkok can be a little too crazy for a young, fresh-to-the-country mind to absorb first up. Plus, you won’t have to carry all your “shopping purchases” around with you.