1. Haha good question but it is a little difficult to describe what kind of a look she would have given the child.

The kind of a look that tells you "I wish you grew up faster" is all I can say I guess haha.

2. I think you misunderstood the sentence.

When my mother gave me a look that made me wonder when I grow up.

The sentence is trying to say that the mother gave the child a look that made the child wonder WHEN HE/SHE WOULD GROW UP.

In other words, The look elicited the child him/herself to ponder when he/she would grow up.

Does that make sense?

3. Yes, the author used the word tangible because she is clear to the author.

The author mentioned this because it has a contrasting effect to the "future was a dip in a road obscured by mist".

To clarify, the author does not know what it is like to be a grown-up while his/her mother is a clear example of a grown-up