Best watch those hydrogen balloons around the birthday cake candles.
For some reason party balloons are almost always filled with helium, not hydrogen. This despite helium's higher cost, scarcity on earth, slightly lower lift, lesser opportunity for amusement, and a permeation rate through balloon envelopes almost as bad as hydrogen's.
Of course, you can always make your own hydrogen balloons at home:
https://www.youtube.com/watch?v=755KpMw_Od8
As for permeation, molecule size is not the whole story. For instance, some scientists were able to make very thin membranes out of graphene oxide that "are completely impermeable to liquids, vapors and gases, but allow unimpeded permeation of water (H2O permeates through the membranes at least 1010 times faster than He)."