Your ideas seem good!
What I have done is work on the same "steering the board with the mast foot" skill set that seems to clear up alot of issues for learning/advancing sailors learning to beachstart.
When they can turn the board 360 deg. in both directions (they learn this in shallow water just deep enough to keep the fin off the bottom) without touching the board, then I have them move into waist deep water.
Then when they can turn the board around with the mast foot in waist deep water, I have them move to
chest deep water.
The skill set required for waterstarting has proven to be precisely the same as the "board alignment" skill
set needed for beach starts.
Yes, the rig handling skills differ (mostly in the angle of the rig to the water), but if they can steer the board to get the alignment to the wind direction right, then throwing the rig up to "catch the wind" and iniitiate a waterstart comes pretty easily.
It works the same as a chest deep beach start.
One of the most common problems is the idea that once you get the board and rig aligned for a water start, that the wind is simply going to pick the rig up.... and the sailor with it.
New waterstarters need to understand that there are times when this is a true statement, and other times when it is not.
When you have the necessary windspeed, yes, you just tip the sail so that it powers up and it will have sufficient power to pull the sailor from the water.
If you have less wind, you need to "help" a bit by "throwing" the rig up into the wind to help it power up.
Extend your arms as far as you can, kick your feet, all at the same time you give the rig a "push" up.
This gives the rig some intertia and will help alot with marginal wind waterstarts.
On the other side of this windspeed vs waterstarting equation, there are time when you have so much wind that you need to keep the sail "unpowered" until you are fully aligned and ready to get on the board.
I've even put my feet in the footstraps with the board tipped on it's windward side becuase as soon as you lift the rig enough for the sail to power up, it wants to rip you from the water and toss you over the leeward nose of the board.
The most important parameter for learning to waterstart is having the right amount of windspeed.
Not enough wind and learning to waterstart just does not work..... even for sailors who've been waterstarting for years.
Too much wind make waterstarting attempts very difficult as you cannot show your sail to the wind until everything is in perfect alignment, and even then you get "tossed" over the board about half the time.
So, make sure you pick the right conditions.
For lightweight kids, it does not take very much wind, so do not select the conditions YOU need, rather the conditions that are safe and easy for the kid you are teaching.
Also, there is more than one way to get on the board.
Many like to put the back foot on the board.... I do not.
Don't like to have my head that low in the water, so I get everything lined up, with no feet on the board, then when I throw the sail up, the rig simply pulls me from the water.
I kick both feet, and just have the sail pull me up enough to get both feet on the board (near the centerline if possible) and then I sheet out momentarily, get my feet set, sheet in and get my harness
hooked, put my front foot in the footstrap, then work my back foot into the rear footstrap.
Hope this helps,