Please forgive me for sounding thick,but do dogs actually need grass?

In my back garden I have a flagged patio and where my grass used to be it is now covered in play bark as my childrens swings,slide,etc are there.
I have a small area of grass and I have decided to have this flagged too because after all the rain we have had it now resembles a mud wrestling ring. :)
The thing is a neighbour told me that dogs actually need to eat grass as it is good for their stomachs.I have never heard of this and was wondering if I should just sow some more grass seed.
My girls are walked in grassy areas but I dont see them eating any as they are too busy running around like a pair of nutters.
They are BARF fed and eat plenty of fruit and veg as part of their diet.
Can I do-away with the mud wrestling ring or should I re-seed?
Sorry for sounding stupid :rolleyes: