I often find it easier to control them when they are in front. I can see mistakes in handling much quicker than having to check back. The downside is that they are at the front of potential cross traffic and stopping situations. It has not been an issue for us but we keep to mostly tame streets with short stints on busier roads.
Mine are small also, but your nine year old is probably ready for a 24" bike. He can probably handle gears but you will likely have to direct him when to shift. The good thing about the bike is that he has 2 brothers and maybe 1 sister that can inherit the bike.
We just bought a 2nd 24" bike because my 2nd (9, average to below average height and weight), was really too big for the 20" and the 11 year old (same size, slightly heavier than 9 year old) has not grown enough to be on a 26" or 700c bike.
The only other option I can think of is adding a Ibert type seat for the 1.5yo instead of a TAB. All the TABs I know of do not have brakes that I know of. Our TAB is geared, but I thought I was certain that the single speeds did not have coaster brakes, I could be wrong though. Unless you have steep hills that empty to danger zones you should be okay as long as you start braking earlier. You can jackknife the train with all that weight but I have only done that trying to stop very quickly.
If you go the 24" route, you will likely be locking the 9yo into riding since I assume the 7yo won't be able to fit the 24" even with the seat lowered. If you get a TAB, you may be able to swap the 7 and 9yo on the 20" with a seat height change.