How is "caballito blanco" expressed in English?

Imagine the situation in which a small child participates in a game with teens or adults. Perhaps the child doesn't know or can't follow all the rules strictly. Or in sports, this child has physical limitations compared to the older, bigger, more coordinated participants. The child ends up playing a special role in the game. In Spanish this is called caballito blanco, palomita blanca, cascarón de huevo, etc., depending on the country and region. The person in this role is permitted to follow the rules of the game more loosely. The term can be used more generally, also, to refer to anyone who's given extra leeway for some reason or other.

What's the term for this in English?

Bonus question: How did the term originate?

This question was inspired by https://spanish.stackexchange.com/q/22950/9385.

Edit:

I wonder if this would work:

wild card

My son sometimes used to beat me at Fish Eat Fish because he played so unpredictably. He was a total wild card.

OR:

stand-in

In horse racing and golf there are well-defined formal systems for handicapping stronger entrants to make the contest more even. In golf this is so well defined that a golfer will 'play off' a specific 'handicap' which reduces as they improve until they become 'scratch' golfers who are always disadvantaged unless they are playing other scratch golfers.

What you are talking about is similar in that the caballito blanco is, effectively, 'playing off a handicap' by being allowed to play to a more relaxed version of the rules than the other players. I can't think of a normal English term for someone in that position but you could try handicap player as in golf. The only problem is that it could be interpreted as saying that the caballito blanco is a handicap to the team.