So, the idea of moving us to the Aldi Family Plan has been put on hold. After the helpful link from @bull and a lot more comparison, I settled on getting 2x Boost starter packs for my wife and I, and an Aldi SIM for the kid. We'll go to annual boost plans (probably the $230 one) and the kid will go on the basic $19 Aldi plan. That still comes out to a lot (like, 40%) less than the Aldi family plan. It was an attractive idea to have us on a single account sharing data and getting one bill for the month, but this is loads cheaper.
Maybe we'll look into it again in three years when the other kid gets to high school/his own phone.