For Druid Wild Shape:
PRIMAL STRIKE Starting at 6th level, your attacks in beast form count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.
For the Monk:
KI-EMPOWERED STRIKES Starting at 6th leveI, your unarmed strikes count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.
The text seems to indicate the ability to bypass magical immunity requires a feature.
Alternatively, I have read that natural attacks and unarmed attacks aren't "weapons," and therefore you should just be able to pummel the Werewolf with your hands if your sword fails:
Melee Attacks (PHB p.195 (or in errata), emphasis mine). The rule on unarmed strikes should read as follows: “Instead of using a weapon to make a melee weapon attack, you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow (none of which count as weapons).
However the errata for Monster Manual I guess clears that loophole:
Global Damage Resistances/Immunities. Throughout the book, instances of “nonmagical weapons” in Damage Resistances/Immunities entries have been replaced with “nonmagical attacks.”
We temporarily ad hoc'd an answer in our game to say the Stirges summoned by the druid were magical creatures(Fey) and hence had magical attacks against the Werewolves, even though they were 1HD(d4). Now I see the druid made the Fey part up, and they are beasts, but oh well.
What is the real rule here?