I would start by removing the "allure" of the computer by allowing short opportunities at home! You can start with 15 minutes daily for each of your children (even better if they want to surf the 'net together) and see how they handle it. If nothing else, get them involved with a fun typing program. I don't think you're "mean", but in all honesty the way our world functions today your kids may have a hard time in a few years if they aren't comfortable on the computer.
Second, I would just openly broach the topic with your in-laws. There's a good chance that they think they are breaking "your rules" by allowing the computer time. It's entirely possible that your husband said something along the lines of "Mom, we don't let the kids on the computer at home and last week they were on yours for almost 4 hours! This week, can we keep it to less than 2?" If you were on the other end of that call, what would you have thought?
I would just call your MIL and mention that you're okay either way, but want to be prepared with games and toys so the kids aren't running around like crazy people. See what she says.
My guy is little, but when we go somewhere that I know he will be the only child (therefore no other toys), I will pack him a little backpack of his own (almost 3 yrs old) with a few matchbox cars, a puzzle, a book and some crayons. While I really enjoy grown-up chatting, my husband and I will rotate playing with him so that he has some interaction too. I have also brought a Disney movie with me if I know that it's going to be a long visit and he will need some "down time".