Looking to Create an Investment Account for a Baby with Specific Parameters

Hullo!

I'm looking to open some sort of investment account for a relative's baby. In a nutshell, I'd like to open the account with a small amount of money (in the realm of $100+ dollars). I will continue to contribute, and urge other family members to do the same (they are interested in and like this idea), in small increments throughout the child's early life (birthdays, et cetera). That way, when they reach 18, they will have a little something to do whatever fits their fancy with. The family that made me think of this idea isn't in the best financial situation at any given point in time, so this would be something that, when the time comes, will be at their disposal regardless of their parents' family finances.

There's a few things that are important to me for this account:

1. The account should only be accessible (from a withdrawal perspective) by the person that it is for after they reach 18 years old, only them, and only at that point in time.

2. I want to be able to continue to make contributions until they are 18.

3. I want multiple other people to be able to contribute at any given point in time until they are 18.

4. I want this account to be able to grow, safely. Lots of subjectivity built into that statement, I know. Open to hearing other people's thoughts and ideas of what that means to them. High-yield savings is safe (over 4% isn't that bad, and is available), but over most 18 year periods, the S&P average has been as well. What do you think would be best? I want this to be an investment for the kid's future on behalf of anyone who is kind enough to chip in, not a gamble.

5. I'd like to be able to share with the parents that this exists. They might even chip in as well, a few bucks here and there, if and when they can. But I want them to have no ability whatsoever to access the funds.

That's about it. Maybe there's important things that I'm not thinking of that I should aware of. If so, please let me know.

The bottom line is that I'm looking to open an account for a baby, that anyone could contribute to, but would be protected from everyone but that child, once they're an adult.

Thanks for reading, and for any insight you have to offer :)