Certainly not Kirchberg that's a business centre. It is not easy or even possible to get down to the Petrusse/Alzette Valley park area by car I think, but please go and try, I haven't been there for years.
Your best bet is go to the country side, along the Mosel river. Try the Remich area or work your way further up towards Grevenmacher, there are lots of places , the local communes cut the grass and keep these dedicated pique nique spots clean and in good condition.
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My kids can do whatever they want as long as they are not Republicans or junkies. That is where I draw the line.