I grew up in Richmond and here are my two cents. Maymont is beautiful in the spring and summer - not sure what it will be like in early March. They have a little petting zoo in addition to cool parks (my favorite as a kid was the Japanese garden). You can do some shopping in Carytown, which is nearby, and there are lots of good places to eat there, including a diner and a Mexican joint called Nacho Mamas that could be good for the three-year old. There is also the science museum, which is in the same general vicinity but might be geared towards older kids - here's the link
http://www.smv.org. Everything I am recommending is within a short distance so you won't have to go very far.
Have fun!