Unless you have an existing condition which means your GP or consultant has already advised against any lifting, the weight of another child isn't going to bring on miscarriage.
It is true that hormonal changes bring about a change in the elasticity of your body and that you shouldn't over-stretch yourself, but sensible liftng with your back in an optimum position, is not a worry.
Once you get much bigger, you may have to look more carefully at ways to avoid lifting too often, but be tactful...
It is not a good idea to blame your growing bump on reasons why you can't pick an older son or daughter up, or the reason why you can't play with him so much, because it will make him or her feel that the new arrival is already coming between you.
However, it is understandable that morning sickness, tiredness etc might compromise the things you can do together.In which case, try to find new things you can enjoy and share. Perhaps a new reading time on the sofa during the day, or making cakes together.