Ha-HA!

I remembered the perfect topic for my definition essay. What is it?

Solve this riddle that I will make up on the spot to find out:

FIRST LETTER
The alpha of the group or pack
The one that leads them all
'Tis pointy when it comes in CAPS
And round when it is small

SECOND LETTER
Riddle, riddle, riddle you
What is it that makes a riddle?
Without this thing it could be middle
Or tiddle, fiddle, griddle, piddle...

THIRD LETTER
A family had children four
The second one was not that bright
Yet the letter that starts this child's name
Will for this riddle shine the light

And if you do not know this group
I'll give you one hint more:
This letter appears in depth and catch
In Mary Poppin's favorite word as well

(Which, spelled backwards, is suicodalaipxecitsilagarfalacrepus)

Have you guessed it?
If you haven't, too bad. I've run out of both ideas for hints and rhymes.