I can see what you're getting at and, when I get to that point, I can't say that I wouldn't do the same. In fact, I will be in a similar situation soon with my nieces and nephews (soon to be six of them!). However, my in-laws will also be in such a situation with my children.
To start with, my mother and step father used to undermine our authority (this was intentional). Also, my mother is a very "superstitious" woman (ie, she takes from a variety of religions, mainly native and some christian with respect towards mormonism, the paranormal, and generational cursing...read "Saved by the Light"). There is no way I want this woman spreading her views to my children. Your children may feel the same about your beliefs (even though not heretical) and you DO need to respect that to a degree. You raised your children and now they are raising theirs. The authority rests first with their parents. I also must be careful not to intentionally undermine my brother in law.
However, if the children ask, there is nothing wrong with giving an honest answer. Just attempt not to make it an attack against the parents. I have dealt with this with other people's children even within my own church. A child approached me one time about my covering (this was the most confrontational child ever...but truely curious). Of course, her first questions were "are you from another country?" and "are you from way back when?" (literally!

). After receiving the negs on those, she wanted to know why I always wore things on my head. I first told her that I would like to speak with her mother. I asked her mother's permission in this case as I would be saying "that the Bible says so" and didn't want to cause what the parents might see as an issue and to give the parents the chance to form their own answer to their child if they disagreed or just wanted to admit to not having studied it yet. The mother was more than happy and we went and spoke with the child together. It was very respectful and it opened the door for both the child and parent to read and discuss what I had to say.
Sometimes there isn't an oppurtunity like that and you think you need to speak up anyhow or just to be honest. In those cases, I would couch my words with, "well, your parents and I disagree on that. Here's what I believe...---...(sometimes adding) but you might want to go talk to your mom and dad about what they think also". This isn't to be universalistic, but to open the door and to show respect and not appear as though you are trying to undermine the parents' teaching. And still, you are planting the seeds that will grow in that child.
There have been times where parents/grandparents have asked that the children not discuss topics of the religious genre as children as a tendency to misspeak and thus come across offensively. However, the same have been willing to speak to my husband or myself with the children in hearing range. It has removed any misunderstandings on where the differences are and opened the door on respectfully stating ones view (and listening to theirs helps me to learn enough that I can adequately formulate my answers without just making false assumptions upon another's beliefs).
I hope this all made sense.