You might be better to join a club that goes out regularly together and can hire boards per session. I can’t see what you want to achieve is possible with that many boards and children, just carrying the buoyancy vests is bulky and awkward. If one of them falls in you are also travelling back with a dripping child unless you are carrying spare clothes too.
I only SUP on the sea, pumping a board up with a manual pump is HARD WORK especially getting the last bit of Psi to make sure the board it properly inflated.
If it’s the exercise aspect you are really doing this for I’d find another activity coz the hardest bit is inflating the board, you’ll need a breather before you actually go out on it 😂 I don’t find SUP massively strenuous (I’ve never broken a sweat) it’s actually more relaxing than anything :-) but it is a marvellous way to get outside and enjoy nature.
SUP looks pretty simple and benign but can go wrong quite quickly. Unless you have experience, understand currents, tides and wind conditions I wouldn’t be putting the kids on a board solo. Not saying you don’t already know these things perhaps you do.
I had three lessons with an instructor before I started flying solo so had some confidence with controlling the board (including falling off! It does happen!), when to know conditions were changing and to head back in quickly etc but as I say I paddle on the sea where it can be unpredictable with changes of wind direction and effect of tides etc.
I take my daughter on a board with me and have a second ankle tether so she is always attached to the board and can’t drift away if she falls in.