So after ardor is out when is the snapshot for the childchain to take place, and how long is the snapshot?
This.
Do I have to hold both NXT and Ardor until goes live (NeXT summer?) to receive Ignis?
Yes, and after that you need to keep NXT, Ardor and Ignis to recive Pomtium:D
ps. ofc im joking but in this crypto Zoo you must be prepared for everything