You aren't alone in the struggle. I have leaned on a good friend twice to help me bury my dogs. They are next to one another but I haven't decorated the site yet(complicated). It's marked & safe.
I was able to vent through the anger & exercise of digging a large hole but the other half needed some support. They were both about 15 years old.
Maybe not a human child but a dear family member nonetheless.
As a bit of personal tragic humor, my sister posesses about 40lbs. of her late husband's cremated remains(he was big) and needs to get some of it into outer space.
There's apparently a company (in Switzerland i think) that converts cremation ash to synthetic diamond.
Best wishes, friends.