What kind of weed killer can be used to kill small oak tree plants from fallen acorns on grass without killing the grass???

B. Buckland
There is not an effective way to do this. The chemical weed killers will kill your tree and the natural herbicides are not strong enough. Some of these may not be coming from acorns. The roots are known to send up little shoots also. Cutting them is a control but not a solution.
Tony M

I've been mowing mine for 10 years. I haven't spent 10 minutes worrying about them.