Hi, Debbie, welcome. As far as fixing cracks in grout, you can always repair grout. Just dig around and make sure that all loose grout is removed, then mix up a new batch and grout the piece again. Seal it with a good outdoor grout sealer. Sometimes it is never again an issue. If it continues to be a problem, you probably should figure out why it's cracking. There are many reasons, the substrate, the mix, the grout lines are too wide, etc., but when I had grout-cracking issues, it always stayed fixed after I regrouted.

Here is a link that might be useful: Silva's mosaic blog