Is it childish for an 11 year old who will be turning 12 soon to use a Smiggle Pencil Box in the color blue and pink with hearts school?

No, it's not childish for an 11-year-old to use a Smiggle pencil box in blue and pink with hearts at school. Smiggle products are marketed towards kids and tweens, and many older elementary and middle school students use them. Personal preference in stationery is a matter of individual taste and doesn't inherently indicate childishness. As long as the pencil case is functional and doesn't disrupt the classroom, there's nothing wrong with it.