My stepson was kinda forced to be introduced to my mom when he and his dad came over to the States in 2007. It seemed to go well enough, and my mom teased him about her being his grandma and stuff, and my stepson seems to like her. She hasn't met my other two stepchildren yet, but knowing my mom it'll be fairly seamless--since we stay with her when we visit the States, it gives both parties a bit of time to get used to each other.
Now my mom asks about my stepson, and she'll tell me to say hi to the kids when she phones up. It was a natural progression I guess, since my mom is fairly easy-going and tends to not be overwhelming or smothering, especially if it's not kids that are her own.
I think in some ways it just happens, and it all depends on each individual child really on how they might react to step-grandparents. If they're shy, they might have a bit of a harder time getting on with your parents, or if your parents are very overwhelming or overly physical. If your parents are like that, I'd maybe tell them to tone it down a little bit, at least with the first visit, and explain to them that the kids might be a little odd or standoffish at first until they get more comfortable with things. But, these might all be things they know already, but it doesn't hurt to discuss it beforehand just to make sure everyone's on the same page.
In the end, it's mostly down to how the kids react, and they can be quite surprising--they can take to your parents like ducks in water, or not, it all just depends. Maybe your husband can also talk to them about your parents visiting, and how they're excited to meet the kids, and how your parents relate to them via you so it's not just a 'whoa, hello strange people' type of thing for them. It's a highly individualized type of situation since there's so many variables at work, but it can be kinda stressful as a step-parent trying to make everything work well when blending the families : /