Taking someone lightly with

甘(あま)く見(み)る means “to underestimate” or “to take lightly”. The speaker warns that someone has failed to recognize a person’s ability, a problem’s difficulty, or a situation’s danger.

Further Reading

Examples

Don't underestimate your mother with 甘く見る

Manga panel from 怪盗セイント・テール showing example of Don't underestimate your mother with 甘く見る.
怪盗セイント・テール » Volume 7 » Page 46

Meimi’s mother stops by Meimi’s room with a couple of movie tickets. She says she and Meimi’s father have already seen the movie, so she’s offering them to Meimi. She says the movie is very romantic, and suggests Meimi has someone she’d want to see it with.

芽美(めいみ):
「マ… ママ知(し)って…」
“Mom, you knew...?”
ママ:
「ママを甘(あま)く見(み)ないことね」
“Don't underestimate Mom.”