Do a child have to be 18 or older to inherent money that a decease parent, has left them?

That's not how rich people pass on their money.

There's a whole industry that deals with passing on wealth. For something small like 5 million (big to me, but small for wealthy people) the most common way is a trust fund.

A trust fund can have stocks, land, cash, bank accounts, really anything a person can own a trust fund can own. The children would be the beneficiaries. And there would be a trustee who's job it is to over see everything.

There are many ways to set up a trust. I'll give you a common example. There would probably be a clause that the new guardian could have money for expenses to raise the children. And pay for schooling, since the kids are probable going to private school. And the kids would be allowed money for college.

For the most part access to the money would be limited until the age of 30 or even 40. This is how wealthy people make sure the money doesn't get used up by silly youth.

And of course there's a full will that says what happens if the children die. Things like how the grandchildren inherit the money.

The trustee is the person who makes all the decisions. They're suppose to follow the terms of the will (or other legal documents. Usually it's all in the will.) But they also have a lot of freedom to do things like make investments.

The trustee should not be the guardian. Putting the guardian in charge of the money can lead to all kinds of problems. It increases the chance of fraud and unintentional misuse of the money. It also increases the chance of just bad decisions. The guardian is emotionally involved. And the children are likely to resent a parent or guardian who holds money over their head.

Neither. The guardian has to hold it in trust until they are 18. They can use it for the care of the minor children. If the parents are smart, they will put it in a trust in the will so that it will be safe until the kid goes to college or until they are 21. (Motivation for college.)

in the US, the inheritance would be held by the guardian for the benefit of the children, probably in a Trust. It could then be used by the guardian for the expenses, etc. of the children. periodic reports to the supervising court may be required, which can also audit the assets and accounts if deemed necessary.

should the guardian take of the inheritance for his/her own benefit, such taking may be criminal fraud and subject the guardian to prosecution, fine, penalty, and/or imprisonment if convicted. [it that case, the court would appoint another guardian for the still minor children and their assets.]

They have to know how to spell inherit.