Weight limit on lifting?

I'm a nanny for two little girls. They are both around 35-40 pounds. I'm currently about 6-7 weeks pregnant & was wondering if there's a weight limit on lifting children? I'm not talking about picking them up to put them on my hip it's more like lifting them up so they can see inside the high pantry shelves and things like that. I am taking it safe and not lifting them high (plus my back aches due to being pregnant).