This condition is called plantar fascitis. I had it for a period of several months a number of years ago. It comes from the tendon that runs the length of your foot from heel to toe being inflammed and tightening up when you're sitting down. Then when you stand up, it *hurts*.
What helped me was each time I went to stand up, whether in the morning getting out of bed or anytime just from a chair, I would stretch my foot (or feet, if both are involved as mine were) out *before* I got up, pushing my heel out and stretching my leg to its full length.
I would then flex my feet forward and back several times before I actually stood up. When you do stand up, give your feet a few seconds to stretch further before moving. That also helps alleviate some of the pain.
My doctor told me to take Ibuprofen, an anti-inflammatory, twice a day for about 10 days. If my condition had not improved then, I was to see a podiatrist (foot doctor). Thankfully, in my case, it *did* get better. But it does take time and a little bit of TLC for yourself to give those tendons time to heal.
I know that time and TLC are not plentiful commodities in the life of a mom, especially with young children, but you've got to take care of yourself first if you want to be able to really take care of your children as you'd like.
Good luck!