What success rate design is the most “fun”?

I’m looking for researched answers here, not just your feeling of what you like best.

I’m setting up a lego based rpg for a bunch of young children, and my friend and I are trying to figure out what “DC” will be the most fun for the kids.

We both feel that a DC of “3” or “4” is good, but are divided on if that should be rolled on a d6 or a d10.

I thought I had read somewhere that 70% success rate is the most fun, but now I can’t find it.

Relevant background: There are no skill modifiers. Things you are good at require a 3 or better, everything else requires a 4 or better. Things that are impossible or things you always succeed at are not roled. The premise of the question is as follows: Always losing is not fun, always winning is not fun either. 50% is the most fair, but because people prefer to win, they would have more fun if they won more than 50% of the time but less than 100% of the time.
I.e. is it better to do this on a d6 with a 50% or 66.66% chance of success, or on a d10 with a 60% or 70% chance of success. (or even a d20 with a 80% or 85% chance of success.. but I’m guessing that wouldn’t be fun)

Each attack does 1 or 2 damage(warrior or mage) and each creature/character has 2-8 hp. Mage spells are move Lego brick, do 1 point of damage with Lego flame, heal, or fly to any chosen spot within movement range.