Unless you're planning on doing one design racing, I can't see why you would want to buy a LT. I think you'll have more fun on a modern shaped board and especially with a modern sail. Since you already have some skills that should come back to you very quickly, I would look at something around the 145 - 165 Litre mark . The more wind you're going out in and the more time you're going to spend windsurfing, the smaller board you should get.
Eg something like these:
JP Magic Ride Family.
jp-australia.com/p/windsurfing/boards/magic-ride-family/or
JP Funster Sport.
jp-australia.com/p/windsurfing/boards/funster-sport/or
Starboard Go or Go Windsurfer
windsurf.star-board.com/products/go/There are other similar boards eg Goya Surf Trainer, Fanatic Viper 75
However, for your 10 year old to windsurf or even SUP, something around the 110 - 125 lite mark would be much better. A fin in the centre of the board or indeed a centreboard will make learning easier. Bigger boards can't be steered properly with a kids size sail and they just feel too big and heavy and unyielding to kids. You could compromise between both of your needs and get a board around 135 Lites which you should still be able to uphaul, especially if it's a wider shape. That size would allow you to progress and have some fun planing and learn how to carve gybe, if you're interested in that. But then having two boards and both of you being out on the water at the same time is even more fun. You can also tow him behind you on a surfboard.
Don't be afraid of learning to waterstart. With some determination it's not that hard (unless you're trying to learn with the Windsurfer LT sail)