I was 90kg when I had Emily (I'm the same height as you). In the 2 months after having Emily, I was fortunate enough to drop down to 74kg without doing anything except sensible eating and breastfeeding. But I couldn't budge below 74kg. 4 weeks ago I joined the gym (Curves) and it's the best thing I've ever done. I had absolutely no motivation to exercise. Even getting out the front door to take the girls for a walk was a hassle for me. But I just got to the stage in my life where I looked in the mirror and absolutely hated what I saw. I can still remember being 57kg only 4 years ago.
Seeing as you have 2 under 16 months, it might be a little hard for you to join the gym, unless you've got your partner or family to mind the kids for an hour at the most each day.
Other things you can try are walking or swimming (maybe taking your older one swimming lessons).

Good luck with it all.