Update:

The Blood Shield Mastery in the offense tree ALSO does not proc when killing spiders. I think I have nailed why the two masteries are not working.

The baby spiders are not considered “enemies” in the game’s code.

I suspect that Kill Streak, Swarm Smiter, Exalt in Death,  Whirlwind of Death, and any other MASTERIES that involve “enemy” kills do not work.

If this is a bug, it’s a serious problem. As it seems these masteries were made with spider’s den (& campaign) in mind, and spider’s den specific champions (AOE champions), rendering the champions and the masteries near obsolete. If you look at the Discord posts in the game’s official channel, everyone’s posts for beating Spider level 20 does not involve the high rank AOE characters anymore like Rae or Queen Eva.  

I am also having this issue. I added this mastery to Zargala, Lua, and Bellower. None of them proc with the spiderlings.

I thought the other "kill matters" skills were proc-ing on the spiderlings, but it can be a little hard to tell with the speed one. 

So basically this skill is about 80% less useful in my opinion. I added it to these characters in hopes of improving my Spider Den teams (I'm still fairly new so my character pool, especially top end champs, are fairly limited). I still have to clear several of the tiers. I will now have to re-evaluate completely and that will end up costing me silver that I desperately need for other things. I am thankful that we have the opportunity to re-allocate mastery scrolls for free once per champion, but this is a feel bad, for sure. 

That being said, if it is a glitch, PLEASE fix this. If it is not a glitch, what's the actual harm in fixing it, so that it does work on the spiderlings? Does it make clearing the spider den 100% easy? Nah. You still have to spend the time and resources to level and gear and mastery your specific characters to make them good for this specific niche dungeon (this dungeon is a fun idea, btw!). 

Another potential option is having a dungeon-specific note on this (and other kill-matters) mastery saying something like "spiderlings only trigger a 2% heal with this effect" or having the Spider Mom have another line in the very wordy (but interesting) passive ability stating that these kill-matter masteries are all halved or something like that. Half the speed boost, attack boost, heal, shield. 

Ultimately, there should be transparency in this situation, either way.

 If it is not and will not be applicable to the spider den, perhaps a free mastery re-allocation to all champions who currently have the mastery unlocked could be issued? Not expecting any sort of major "refund" but maybe something as small as that could make player base/customers not feel totally crummy for being unintentionally (I assume) mislead/misinformed about this.