By us the minhag was coins ‘davka’. That means it started with pennies then increasing with the yrs. After a quarter came 1/2 dollar coin then one dollar coin. Additionally it stopped once i was old enough to earn my own $ with the explanation “I was no longer a child and understood the value of mitzvos without it”.
I was also told, as mentioned earlier, the concept started in order to encourage the children to learn(like the concept of allowance). And every time i got a coin i was told “This is a reminder to learn”. Which is not doable with presents. Additionally, the money can later be used for anything including tzedaka of my choice. A present is much harder to do a mitzva with unless its a sheep being saved for Korban Pesach.