You can put down mouth balls to keep cats out of flower beds. You do not want to let the kids play with those if there are any around, so be careful. Or you can give the cat an alternative, such as a sand box or a garden bed of cat nip just for the cat.
The best way to keep the cat out is to put up a small wooden fence around the flower beds. If that doesn't work, try putting out some red peppers or orange peels in the bed itself. This is said to keep cats away from flowers. Good luck!