With Kate and her work, she HAS done some good stuff in past, the Hold Still project was great, the Heads Together campaign really did mark a change in coverage of mental health, she launched Mentally Healthy Schools which seems to be a great bank of resources for schools re childrens mental health and as per their annual report the foundation does distribute a lot of money in grants to various places. I believe they gave a lot of money to set up the shout text service too.
The problem is that early years isn't a quick fix. You're not going to wake up after an awareness campaign and suddenly everything is fine, it will take decades and a lot of funding to see real change. I'm willing to give her the benefit of the doubt and see where she goes next/in the next few years and I do think there is value in having such a prominent figure as the chief champion of the topic.
Whilst I don't buy it's her life's passion I also don't necessarily think it's a puff piece/PR project to give her something to do, I think she does genuinely have an interest and want to help, but it's not like climate change, mental health or womens rights where you can see tangiable results from smaller projects, this is going to need long term funding and investment.
One thing the royals have going for them is their convening power. There are many brands who would jump at being involved with a royal project who wouldn't look at it twice otherwise so she could do something like she did for the baby banks (which was a great scheme) on a bigger scale, getting private sector investment.