In short:

Slightly, if the kids are not used to masks.

An early study–a few weeks into Italy’s lockdown in 2020–showed that masks significantly decrease most people’s ability to decipher emotion. However, a year into the pandemic, a recent study, published in JAMA Pediatrics (the highest ranking pediatric peer-reviewed journal in the world), showed 276 children aged 3-6 a series of pictures of people’s faces, some with masks and some without, each depicting various emotions. The differences, while statistically significant, were effectually quite small. The authors concluded that, with extended exposure to masks, kids became quite good at distinguishing emotions, even in masks.

Predictably, research has shown that those that struggle with empathy and reading facial queues as is, such as those with Autism Spectrum Disorder (ASD), tended to be worse at recognizing emotion in masks than neurotypical children and adults.