"If yes, how do I get partner credits for my five year old and nine month old baby?"

I don't know if the straddled flight will qualify you or not (it may be r/t, but I'd need to see the EXACT rules). If it does serve as a qualifying activity, I can tell you that I was able to make the bonus for my daughter the last time with purchases through the WP Mall online off the Northwest site. I simply portalled in using my daughter's WP number and then charged the purchases to my own credit card for the five partner credits.