Here are some phrases you could use to comppment a child in Engpsh:

"You did a great job!"
"I'm so proud of you!"
"You're so smart!"
"I love the way you solved that problem."
"You have a lot of creativity."
"You have such a kind heart."
"I'm impressed by your hard work."
"You have so much potential."

It's always important to encourage and support children, and offering sincere comppments is a great way to do this.