I'm going to jump in on this one and say way too much because my mom came out to us the WRONG way when I was about 14. That is, she met a woman, dated her for about 15 minutes, and then had her move in with us and tried to let her coparent. Not. OK. It caused all kinds of drama and hurt feelings that we are even now still dealing with (though I love her and we have tons in common and she's been a great mom except for whatever the hell was going on that one year).
I think you have two options.
The first is to wait until you do have a girlfriend--a serious one, someone you're thinking about long-term stuff with--and then tell your kids first. That might be easier because it's more concrete to have an actual person for them to meet rather than just the idea of one someday. I think this is the way to go, personally. You can answer lots of questions that they and your MIL might have by saying that you weren’t expecting to find your girlfriend, but she came into your life and you are in love and happy for the first time since your husband passed. It’s harder to be nasty about that (though she may try anyway). I bet your encounter with the soccer mom can be interpreted as you saying that you are not interested in dating, since if you haven’t been seen as queer not everyone jumps to that conclusion, so you can still wait if you want. Once you’re out to them and they’ve met her GO SLOW. Slow. Wait a long time to have her move in. Spend more time with just your kids than you do with her or with all of them. Make sure they see you repeatedly choosing them and their happiness over her and yours (you obviously will still make choices to keep your relationship happy and healthy, too, but they need to see that when push comes to shove they win almost every time, over and over and over again). Anyone you date might become part of your life for the long haul, but your kids will always be there. They’re old enough to be watching you and they will remember things that you have forgotten long ago, especially if it’s tied to something this major in their lives. Like Outdoorsy said, a counselor might come in handy here, but don’t push them if they don’t want to go—it’s not for everyone.
Your second option is to tell your kids now that you want to date women. If you go this route, be careful. They might get more flak from their dad’s family and the greater public if their mom is a lesbian but they have nothing positive to say to feel good about their family. No stepmom means no good memories or fun times to hold as an example of how it is a positive thing. I understand needing to be true to yourself, but in this case, I think that the time for it may be when to deny yourself means hiding an actual relationship, not just a feeling. Your kids having a happy childhood (and by that I mean one where they don’t hear a lot of nasty comments about their mom from their paternal relatives) should outweigh your need to be true to yourself until you have someone special and want to be able to be together openly. Then I think the balance shifts and it is more important for your kids to see you being brave and honest even if it gets hard sometimes than it is to try to protect them from the world. It’s a fine line, unfortunately.
Whichever way you choose to go, when the time comes be honest with your kids about your present situation, not your past. What I mean is, tell them that you are interested in women or in a particular woman. You don't need to tell them that you weren't hot for their dad--you can just say that you loved him very much and that you miss him and so on, but that now you are looking for someone else (and of course all the comforting things you need to say about his always being their dad, etc). It's not going to help anyone for you to explain that your marriage was just a friendship at the end, and since he's passed away, I'd go ahead and leave it until your kids are older (like, out of high school) and start asking you more pointed questions. They will, it just takes a while to get there. Stick with how much you loved him until they're at least mostly through college.
Finally, keep in mind that whenever you tell your kids you are telling everyone you know. It’s not good to ask them to keep secrets for you (and they won’t anyway). If they ask you to keep it a secret, refuse. My sister tried that route and it made her high school experience much more difficult because she had to keep lying. Once you are out, be out, even if quietly.
I’m sorry this is so long, and so opinionated. I never minded that my mom wanted to be with women—actually I think it may have been easier for me, since I’m such a daddy’s girl, to not have to let another man step in and try to be fatherly towards me. I did mind very much that she put her happiness over mine and my sister’s, and that she wasn’t able to see how upset were. She sees it now, and we’ve made amends, and I love her wife. I think you can find a way to do this and be happy and get through your MIL’s comments without too much trauma, but only if you make sure your children have lots of good experiences to buffer any nasty comments that come their way.