Wow! Sounds like you must be really busy with the 3 little ones! I only have one boy, 3 years old, we started prenatally with lots of reading, I still read to him a lot (about 3 hours a day at the moment) and this has been the basis of our learning throughout. Other than that, I’ve mainly used BrillKids Little Reader and to a lesser extent programs like Readeez, Tweedlewink, BrillKids Little Musician, BrillKids Little Math, Alphablocks, Number Crew etc. My approach has largely been input based for the first 3 years, but for the last 2 months after his birthday, we’ve started Kumon worksheets, playing chess games, and doing more structured activities where I can instruct him and he follows “orders” (as a tot he would NEVER do this, but he grew up a lot around his 3rd birthday.)
My boy’s development is strange in the sense that I would give him tons of input and see no signs of him learning anything, then all of a sudden it just all comes out at once. He didn’t talk at all until about 22 months, when he suddenly talked full sentences in 2 languages! Didn’t walk until 15 months, but could run soon after! He does this sort of thing all the time. Despite a crazy amount of reading instruction and lessons, he’s only just started sounding out simple words and still finds it very difficult, but I half expect him to just take off with his reading at some point just as he’s done in many other things.
We also neglected math, we tried the Doman dots method when he was a baby but he never really took to it and I’ve never really tried other method up to now, we’re doing Kumon math and I’m very happy with his progress so far (mainly counting up to 30 and back and writing numbers.) He did pick up on some basic math concepts through everyday play, smaller quantities, fractions, money, time, etc. I think he gets adding and subtracting but he doesn’t like to be tested and often gives incorrect answers on purpose (in general).
Your eldest might have missed out a bit, but he gets the opportunity to see his younger siblings go through an EL experience. Does he help you teach the little ones?
I’m no EL expert and I’m sure there’s many on the forum who can better help you with math ideas, but I would give Little Math a try for your 8 month old. Some kids fall in love with it and others never like it, but giving the trial a go can’t hurt.
All the best