Hi.
No one believed me either! Like you I started raising concerns about my DS at the 27 month health check, at that time my main concerns were his motor skills and balance, but we very much got the 'wait and see' response. DS is very bright and has good speech, but there has always been something a little 'off' with his understanding. School did not see a problem, paediatrician felt 'all normal', tests eg eyesight, bloods, physio all fairly normal (had to attend the eye clinic multiple times due to his refusal to cooperate).
I know girls are supposed to be more difficult to diagnose, but DS was a raging banshee of a small child, he used to attack and bite me constantly - his behaviours were not subtle! In school he was fairly compliant, but afterwards his meltdowns would be horrendous, yet school would tell me 'it's normal to be tired after school' etc etc. Eventually took him to be assessed (privately) aged 9 and was diagnosed with ASD. School still not terribly helpful, but it has helped DS with his understanding of himself. It would have helped me to have an earlier diagnosis but I don't think it would have made much difference in terms of support unless I had been prepared to change DS's school.
Perfectly possible to get a diagnosis without school support, as many people have, but I suspect more difficult.
Does your daughter have sensory issues? My son has and I massively underestimated how much they were affecting him, especially at a younger age. In terms of the 'wait and see' approach, I think it is reasonable to a point (as some things get clearer with age), but I think with hindsight, that DS should have been assessed at 6/7 years of age, as I think after that things become harder to unpick as they get older. Things have got easier for us as DS got older and the violence towards us is much less of a problem.
Interestingly, the pre-school did see a problem and we were referred to some sort of early years service who assessed him and also agreed - I have a weird memory of being told that as an educated, articulate woman, I needed to 'make them listen to me'. Ah, if only it were that simple!