Although you may be lovingly aware of what your wife has given up for you, just to be with you- And although you of course tell her alllll the time about how thankful you are that she joined you and your children instead of starting a fresh life with someone else- Think of getting her something from you for Mother's Day, beyond just the children's gifts. And if things are difficult in your marriage right now, lately, or this past year, then this is direly important.
(And yes, definitely, the children should also be giving her something as well, as belief follows action...This shouldn't even be questioned. Although your wife is not their "real" mom, she still does things for them, often playing the mom role in the absence of their mom, and thus deserves the partial mom title, whether the children agree or not. Children don't agree with most as it is, so don't let them question this. And you current wife performs "mom" in a different way, which will always be seen as "not mom" for the rest of their lives despite the truth that she is still being a "mom" to them during your custodial time.)
Make the gift from you meaningful, as she chose to be a stepmom because of her love for you. She chooses to put her neck on the line daily, risking interactions with your ex-wife, getting attacked by your ex-wife, and also getting attacked, ridiculed, and even humiliated by your children in front of others...She watches as the children she wished were hers, with you, get more and more distant or fight to even be around her. So think of what she gave up for you, and then make this gift meaningful in response to that.
Given that each woman and couple are different, there are "big" gifts you could give her that specifically relate to her. I can't tell you what those are, whether it would be alma mater gear from the school friends she moved away from, a trip to see her friends or family, or replacing something she lost when she married you and moved. Maybe a trip to somewhere you went together, pre-ex-wife-and-kids reality. There are probably many thoughtful, specially meaningful things you could give her in honor of her dedication to you and your kids despite it all.
But beyond those entirely thoughtful ideas, consider de-stress gifts, such as spa day- alone, with friends, or with her mom or sister. If spa certificates or reservations are too much for your budget, there are spa-like gifts out there, such as massage oils, aromatherapy, soothing lotions, and soft, sensual robes or super soft pajamas. Make it clear that you are trying to help her relax, and that you want her to have some down time- that it is OK for her to have some rest and self time.
What is her favorite thing to do? Buy gear or accessories related to it, set up a day of it, or buy gift certificates for her to do it.
What is her favorite store that you and the kids just can't stand and vocally complain about? That's the store you should buy her something from or take her shopping at.
What is the restaurant that your ex tells the kids is too expensive or too fancy for you two to go to? That's where you should go with her, on a night without the kids- or with if you purposefully want that report to go back. What is the restaurant that your kids have turned into a place you can't go anymore, for whatever kid reason? That is also another candidate for dinner out.
What is something that you two haven't done together in a super long time? Maybe it was just lunch together, brunch, hiking, sitting at a lake, or some activity- Do it. Plan it. Make it happen on Mother's Day.
Mother's Day is a really, really rough day for stepmoms. I do occasionally read how a stepmom doesn't care anymore. I only partially believe it, because I know how I feel and I read a great deal many more Facebook posts every Mother's Day from stepmoms talking about how horrible they feel and how they wish it all can change, which they know it won't, and that they stay anyways. In fact, every Mother's Day, in every Stepmom group there is, there's at least one discussion post from the moderator asking if everyone survived. Beyond that, there is article after article posted about how to "get through" Mother's Day for stepmoms. You may not know this, because you're not allowed in (they're private and closed, to ward off court-centric ex-wives), so I'm letting you know that how your wife feels and acts is extremely common. In fact, I strangely have the same conversations with stepmoms over and over again, and each time the stepmoms involved are just amazed that others feel the same way and think the same thing. Despite our differences, your wife isn't especially crazy or difficult. she's just in an especially difficult situation.
Despite whatever happened this year, despite whatever your wife (the current one you want to keep, remember?) did or didn't do, despite whatever anger has been between you, and despite whatever your kids hold against her, she is still the woman you married and enjoyed pre-kids and your ever-present ex-wife (whether nearby or as channeled through your kids and your phone). She is still there, but she is in a confusing state and strange world that has until recently been kept hidden and hated. She is only responding as she thinks she should, and trying so hard despite wanting to give up.
You are her encouragement, and she needs you. But most of all, she really needs you to make her feel loved on Mother's Day- and not necessarily (but maybe) in the Valentine's Day way. Figure out what is missing now, what she really needs, and what will honor her attempt to be the third wheel in your family experiment.