re: How hard is the jump from one child to two?

I'll add for OP, I think continuing your level of activity if you can will be helpful.

We do a lot. We go out to eat a lot with a 3 year old and 2 seven month olds. We all go everywhere all the time. But what I've found is it's not THAT much easier not to do anything. Thus far, it's made our kids flexible. Getting three small kids in a car is probably the worst part of my life, but it keeps them occupied and keeps you in the practice of being active and social and I find it more rewarding. The days and weeks pass a lot quicker and we don't really feel like we've missed out on much.

Everyone being mobile is the next phase and it's daunting, but we're going to try and keep up the pace as much as we can.