I have four children. Ages 5, 3, twins 2 ( one of the twins has down syndrome). Do I just do the same lesson for all the children?
Would depend on the lesson and the subject I would think as well as your childrens individual levels and abilities.
That is my point. All 4 are at different levels. The book does not discuss how to change the lessons for different ages and abilities. Day one … Lesson one.
If you are referring to either the Doman book or BrillKids reading if they are all non-readers you would do the exact same lessons for all.
If I was starting out with older children for reading, ( which I did 
) I would start LR at day 20 ( which I didn’t but should have :biggrin: ) for the older than 3 kids, skipping the first 20 days of words that to be honest this age group don’t particularly relate too as much, and getting strait into the pattern phonics. Catch the little ones up on the lessons in between family lessons if you want to.
To adapt lessons to the older kids add words like " bulldozer" to replace words like " bottle"
Also for the older kids especially the 5 year old I would add in some phonics activities into their everyday play. Something like fridge magnet letters to build words or just mentioning beginning and ending sounds in words around you. " you want to play cars? car c…a…r…"