Would it be cheaper/easier/better to have more health visitors or similar to help the parents who are struggling with things like toilet training their 2-3 year olds? I know when my now 18 year old was little we had HV doing regular development checks, giving out advice etc, and if course we were at the sure start centre most days so could have support from the staff there as well. The support available got less with each of my children and since covid it's gone downhill again. I had lots of helpful advice (and some not so helpful) about potty training, including putting the child in pants under their pull up to start with so they can feel what it's like to be wet.
Only one of my dc went to school in nappies but he would have had a note from the consultant if necessary. Although in the 1990's he probably wouldn't have survived. My 2nd child is still waiting for an autism assessment aged 16, despite me continually raising concerns since he was 1 year old, and him seeing many different health professionals when he was little. He was dry about 6 months before going to school so didn't need support on that front but that's just an example of how children can have SEN but not diagnosed for years.
Personally I think parents who don't potty train their children because they can't be bothered won't be trying to get their children into special needs school. That requires far more effort than potty training.
I've just had a thought, but do you think the increasing amount of nursery fees mean that some parents think that outsourcing potty training should be included in the price? I mostly only used the funded hours so had fairly low expectations on what I thought nurseries should do. I know I have higher expectations for an attraction to be good if I've paid a lot for entry. Also if I've paid £4.95 for me (yes that's the adult price) to sit and watch my dc at soft play then I'm expecting a clean floor, comfy chairs and decent drinks/snacks available. If it's free for parents to attend and £7 each for the dc then I'm happy with a lot less.