Before anyone can accurately help with your question, we will need to know more about the way your pain comes about and how it manifests it self. for example:
Is the pain worse when you go from bending over 'to' standing up straight?
Is it worse when you go from standing up straight 'to' going backwards into extention (eg: reaching up high)?
If you are lifting light objects is the pain the same? or does it hurt 'more' when lifting heavier loads like your Children?
Do the pains tranfer (spread) to other areas such as the legs, back, neck or arms?
When this pain happens to you next, take a little 'time out' and accurately write down what you feel, where it is and how it makes you feel afterwards. It may be a very minor thing, but until you can accurately describe it, we (on this forum) can't really offer you any useful suggestions.
Sitting, posture and excersices are well covered by loads of Members on this forum, have a look around for previous and matching posts!
Best wishes
SPINELF