There is a way, but it takes some time.
Get some string and thread bits of paper ( colourful sweet papers are good ) and put them across the new lawn at about 30 cm intervals. The noise of the paper deters the birds and the approxamety* of the string deters the cats.
* spelling mistake!!
Similar to above, I tried stringing loads of CDs (I kept all the freebies that came with magazines etc) across a newly seeded patch and they appeared to keep the birds away with their continual reflections as they moved in the breeze. I don't know what the effect on catswill be. Andy