Zylie's Secret World (ZSW) is intended as a low difficulty server that aims to include things appropriate for mid-age (recommended 8+) children, including several decorative mods, a creature mod (DrZharks) which includes unicorns, Minecraft 1.12.2. It also includes several UI enhancements such as realtime mapping (if running against a dedicated server) via Journeymap and JEI/WAILA to help with the much increase number of items.
It also includes Thaumcraft as a harder challenge for more dedicated or older players to progress on. By itself, Thaumcraft is easily enough to entertain a good player for dozens of hours. Thaumcraft is predominantly an "elemental magic" style mod, but far in the mod there is "eldritch magic" that could be scary for younger children. However, depending on the interests of the younger players (i.e. possibly only interested in building houses & flying ponies), they might not make it through the 5+ hours of progression required to unlock Thaumcraft's Eldritch magic in the first place. Either way, be aware.
For younger players, I recommend giving them invincibility via "/effect playerName". Resistance 5 should be sufficient. This command should work, ("/effect @a 23 1000000 20 true") though you can also see https://minecraft.gamepedia.com/Commands/effect
Major mods:
* Chisel (lots of decorative blocks)
* Biomes O Plenty (more biomes and interesting worldgen)
* Minecraft Comes Alive ("Sims in minecraft")
Note for Biomes O Plenty: you must set "level-type=BIOMESOP" in server.properties before starting the server, or delete the world-data and restart the server after setting it. See https://www.curseforge.com/minecraft/mc-mods/biomes-o-plenty

