The custom is to give 3 halves; in Israel 3 half Shekels, in the US 3 half dollars (for a total of $1.50).
The reason for the triple halves is that in פָרָשַׁת כִּי תִשָּׂא that discusses the מַחֲצִית הַשֶּׁקֶל it says thrice תְּרוּמָה.
These 3 halves are given before Megilla reading - and are then distributed to the poor.
Children under Bar/Bat Mitzva are exempt; some opinions exempt them until they turn 20. However, once a father has given 3-halves for a child - even once - then he has to continue doing so yearly.
(The above is not to be confused with the Mitzva of מַתָּנוֹת לָאֶבְיוֹנִים that must be done on Purim.)
Source: Kitzur Shulchan Aruch 141:5
- Danny Schoemann
Monday, 11 Adar 5775