Normal lang yan, mommy! A lot of kids that age still drool a bit when they sleep, especially if they're deep sleepers or if they sleep with mouth open, ang aking son naman ganyan pa rin kahit 7 na siya. The bubbles are just from the saliva mixing with air, nothing to worry about, pero if it's excessive or may other signs like difficulty swallowing or speech changes, mas better to mention it sa pedia just to be sure!