I have the opposite: my mum is stricter than I am and complains I'm indulgent merely because I discipline in a less obvious way.

You could lay down a few groundrules with your parents of lines which they must make sure are never crossed, but otherwise, when your children are with their grandparents they will know that different rules apply. This does not mean that your own discipline will be undermined; children quickly learn that different people have different rules and boundaries and adapt accordingly.