Toddler and Children hair recolours

Are you asking about creating new hair or enabling the game colors?

Sims 4 Studio has tutorials for creating CC. You will need to sign up for a membership but, it's free and will give you access to their program which you will need to create new content.

The Witches Lair has added unnatural colors to many of the game hairs (up to Seasons, I think) and enabled them for genetics.
http://thestuckylibrary.tumblr.com/

and there are many CC hair available with unnatural colors:
http://sssvitlans.tumblr.com/hair

I just want to enable the unaturals colours for toddlers and children

Pyxiidis includes a method of adding genetic unnaturals for kids and toddlers in this post somewhere. It's one of the extra packs on that page. It explains in the post. I know because I had to make sure to avoid it. I like unnatural hair, but only on the chosen few XD

http://pyxiidis.blogspot.com/2017/07/midnight-hair-defaults.html