Using Power Good pins for sequencing
Rather than build your own discrete solution out of transistors, I'd highly recommend using a reset / supervisor IC for this. I used that approach when I had to solve this exact same problem on a design in the past. They do the exact job you're trying to do here -- wait for voltage to stabilize, then release the /RESET signal after a fixed amount of time. Just hook that supervisor IC up to the regulator's output, and use its /RESET output as the ENABLE input to the downstream regulator.

They're tiny and cheap -- I'll wager a smaller and cheaper solution than a discrete circuit. A few examples:

Some SOT-23s from ST 

TPS3836 from TI