My doctor gave me a 25 pound weight limit with my first baby. With my second pregnancy, I stuck to that weight limit, as my "older" child had only just turned 17 months and was about 20-21 pounds when his little brother was born. Now, I have a 6 month old (18 pounds) and a 23 month old (26 pounds), and sometimes, I have to pick BOTH up at the same time. This is an emergency situation, but it has come up about 8 times throughout this pregnancy, now at 17 weeks. It hasn't caused any problems so far, but my doctor told me that I definitely should NOT be picking up both and should use my older child as my weight limit, expanding my limit as his weight goes up. In THIRD tri, lifting heavy objects can cause issues with the placenta, so it's not worth the risk. However, it can still cause issues before then and/or can cause real problems with your tummy muscles as your body is growing.
I recommend starting NOW not to lift anything over 25 pounds and then talking to your doctor about it later. (If you hold a certain child regularly who might be over 25 pounds, your body may or may not be used to it, so just listen to your body and use precautions.) However, there's no reason why you can't hold a child of any weight while you're seated. Have the child go up to you for a rock or a cuddle if needed. Just to be safe!