Just a thought....
For your eldest and Aspie kids call/text is probably the BEST way to go (Oy, ESP if your Aspie is anything like my son's bestie or my neice who are both technowhizzes who hacked the computers in their smart phones), but you may want to reconsider for your ADHD kiddo. Totally get if you need them all to match... But just to consider;
I'm ADHD-c as is my son. I take pictures if all my important paperwork (so I never lose it), my homework assignments, and things I just need to remember visually. My iCalendar has EVERYTHINg on it (with alarms), my notepad has dozens of notes. My voice notes I use to record lectures, assignments, reminders, and verbal instructions. My alarm clock is set to music to wake me up over the corse of an hour without annoying everyone else.
My iPhone has been the BEST ADHD "look normal" tool I've ever come across. I've never been more on time, organized, or (insert word for 'not forgetting important things, or key details) in. My. Entire. Life.
My 10yo ADHD son has had his own iPhone for several years. He's never had to bumble along without one, really. I WISH his school would let him use it for notes in school. I'm working on IEP'ing that, actually.
They're dirt cheap. A previous gen iPhone is $1. And only $10 a month in a family plan. You can SHUT OFF every single feature you don't want to allow in parental controls (also limit - by use/type, time limits : including multiple limits, Luke not from 8am-230pm and not after 10pm until 6am-, age, etc.)
What's restricted doesn't even show up in the screen. What's limited (like if you allow g rated movies only, and only from 4pm-6pm) still shows up, but won't connect until the limitations are met.
Huh. HONESTLY.... Massive restrictions on 2 phones, and then ADHD tools allowed on phone 3 would let them all 'match', but have different things specialized for each kid. All for $3. Plus the GPS and 'find my phone' would come with, unlike burner phones.
Just something to consider.