Somehow, it doesn't take update the LESS file in local folder. I just manually updated this file:
/local/less/themes/number3/variables-custom.less
by changing this line:
@font-family-base: @font-family-sans-serif;
to revert back to default font-family.
This update is in a LESS file so you will need to backup your site first, then go to Backend >> Extensions >> Templates >> JA Kidscorner template >> click LESS to CSS button to compile less to css so you can see the change.
Regards