Mother's Day Ideas: Stepfamilies
Where stepparents are concerned, there is always that shade of gray for kids: "They're not my mom/dad, yet they sort of do things for me when I'm there."
My recommendation is that your kids do what they feel comfortable doing to recognize the mom-like role their stepmother plays when they are with her.
If your kids are young, they may want to make her a card or gift. If they are older, there are great stepmom cards in the stores now, and they can give that along with a small token of their feelings for her (perhaps flowers, a plant or their school pictures).
Being the stepparent can be a very precarious position. Chances are, the very fact that your kids acknowledge her on Mother's Day will mean a lot to her.