Minecraft – the RAREST thing to spawn in Minecraft

Spider Jockeys are simply Skeletons riding Spiders, and I know for a fact that Skeletons can't shoot their arrows through glass. If they can, it's most likely a very rarely occurring bug – the fact that there's no citation for that point makes it hard for me to believe. It's not something you need to worry about. (Also, there's nothing on the Spider Jockey page about being able to shoot through glass.)

Glass is just as good as any other block when it comes to safety. The only thing you might worry about is if a Creeper were to explode near your windows, the glass would absorb less "blast damage" and more of your house may get destroyed, as glass has a very low blast resistance rating.

If their Minecraft PE is 1.6.1 or later, I am 100 percent sure that your kids will just have to wait a bit. (About a month in Minecraft days) and if there are few enough remaining animals, new ones will spawn. Eventually. Some of the new spawns will be babies. This unfortunately will wear off in time though.

I know this because I spent hours fencing all the animals in my world. No more spawned for about 3 (real) months. My brother joined on multiplayer and immediately proceeded to kill all my sheep. I was very angry. After a while I went outside and was logging, when I actually saw a baby sheep spawn out of nowhere. Then a cow and a pig. Then a baby chicken. Etc. I checked this on every site I could find on this topic. 8 out of ten said the same thing. Good luck!