Lifting things

So I'm only about 2 months now but I nanny for twin girls (they weigh about 20 lbs each now) I'm constantly on the floor with them (which kills my back, it's hardwood not carpet) and I'm always having to pick one (or both) up and carry them around as sometimes it's the only way to sooth them. I would say definitely that more than half the time I'm with them I am picking them up and carrying them around- is this safe during pregnancy to be lifting and carrying so much? At what point should I tone it down? This is my first pregnancy so I'm still trying to figure all of this out :p