for me, even when I am single that time, I already give parents, grandparents, so I just continue the practise.
for my dad (mum passedaway), usually the amount of give, is either 1 to 2 X of his monthly allownace (like bonus like that). It all depends whether he "good" behaviour or not for last yr, like got go out to gamble, anyhow spend money etc.
Last time I used to give my grandma about $1k, but my grandma (grandpa passedaway) will usually give me back an ang bao, they will like keep about $50 to $100 and returned me the rest.
even my brother and sister, cos I know they have financial difficutly, tho they are married, I used to give (this yr not giving, cos they fought last yr becos of my ang bao, fighting who I cared more to give more, bo pp).
as for children, all children that I see and I will try to give, only differences is the amount:
passerby kids & neighbours kids = $2 (just to have fun and make them happy)
relatives' kids (regardless whether my cousin / yee child) = $4
(approved by granma, cos she said, if too young, give too much, they also cannot appreciate, if too old, give too much for what ? kekekekek).
but for my sister's kids who are staying w me, I will give about $18 or $28, depend on my mood.