I love it when I can find a good resource for informal learning. I found a SlimeKids website that offers several games to help your child practice grammar. These games tend to be best for kids who have studied some grammar already which would make them more appropriate for grades 4 and up.
They have the following games:
Word Invasion: Help the octopus get rid of jellyfish by selecting words with a certain part of speech
Verb Viper: Feed the snake by selecting words with the correct subject/verb agreement
The Apothecary’s Cellar: Help get the ingredients for an elixir with prefix and suffix practice
Grammar Bee: Capture words by highlighting them before the bug gets you
Root Collector: Select letters to identify a word root before you run out of moves
Which one was your favorite? I liked Grammar Bee.