I think we need far more openness and honesty generally about the challenges of having children with disabilities.
I had dd in late 20's, she has a physical disability that was I now know pretty bloody obvious from the start to anyone with any knowledge of the condition and honestly I feel it should have been dx far earlier than it was. She was finally dx more as a result of luck than anything else when she was almost 12.
Once we had a dx and I was able to learn about it so so many things finally made sense!
My dds disability is relatively "mild" at this stage though likely to worsen as she ages.
But it has still caused her distress and suffering including pain and I've had a battle on my hands getting school, family, friends, adults in charge of her care at clubs etc and even hcps to take it seriously!
To the point she has ended up in hospital twice unnecessarily because of Gp's not listening and dealing with related issues appropriately and not even bothering to so much as Google the condition even though they admitted they weren't "familiar" with it.
I've a few friends with children with learning disabilities and I'm in awe of the fact they cope at all! One has a child with autism who is barely verbal at 9 and sleeps maybe 2-3 hours a day and can't be left alone the rest of the time. Another her child has major heart problems as part of the condition and they're in and out of hospital constantly, he doesn't understand that he can't over exert himself and so his parents have to manage that and physically prevent him from running etc.
Gradually we are seeing via various media more people with various disabilities but it's very much (and I understand why) the "presentable" face of disability, people able to engage with others not close to them, to communicate fairly well and inoffensively, who don't look particularly physically affected etc.
It's only in rare documentaries that the more severely disabled are seen (and that's ethically problematic) and that's a mere glimpse into the 24/7 demands of having a severely disabled loved one.
We're possibly looking at at least another 3 years probably longer of this current govt who are massively cutting funding and support for the disabled and the Nhs. The economy is being run into the ground so that massively reduces contributions to charities...
Honestly op do you even know very well anyone with a disabled child? Have you ever spoken honestly with them about what that's like? Maybe read some of the threads on here by families dealing with such issues?
Because you just seem to think your family will "cope" magically, that it wouldn't have a massive impact on your time, energy and finances and your ability and availability to parent your older two - a frequent issue I've known of in families with a disabled sibling is the "healthy" siblings feeling neglected and ignored.