The easiest way to level these characters is to have a higher level character kill certain mobs for you while you are in the party. Their are so-called power levelers (plvlrs) who will do this for in-game money or you can find me and I'll do it for free, if I have the time. The fastest mobs are the Cursed Knights in Flaming Mine (I can help get your characters here to if you need me to), but there is often a full exposition here of babies being leveled, often by a paid plvlr. If you can't get a spot I suggest having someone kill Guardian Masters for you. This is still fairly fast and there are several spots this can be done at. This is my favorite way to level. It isn't as fast or exciting as FM, but it does the trick.
If this isn't an option for you, or you'd rather take your time and experience the various areas of the game, then there are quests you can do at this level. All the regular quests are fairly easy and fast up to about level 30. In fact, in my opinion, this is the most fun way to level especially if you have a friend or two to level babies with. Below are a few repeatables you can do together to make things faster, especially after level 30, if you want 😉
Repeatable Quests (level 1-39)
- Despicable Little Ones - Little Hobs - Level 10
- The Mini Greenky - Mini Greenkies - Level 11
- Disease Carriers - Ratman - Level 12
- Medical Herbs - Ratman with Herb Leaf - Level 12
- Working for Karl - Broken Blade from Hooligan Phinoflies with Copper Ore (NQ and LQ) - Level 12
- Clam Chowder - Boogy - Level 13
- The Apple Thieves 2 - Boogy with Boogie's egg from them - Level 13 - Available after completeting The Apple Thieves 1
- Joker Kebings 2 - Joker Kebing - Level 18 - Available after completing Joker Kebings 1
- Speedy Boogy 2 - Speedy Boogy - Level 18 - Available after completing Speedy Boogy 1
- Herb Plant Set 2 - Herb Leaf, Herb Stem, and Herb Root (from gathering herbs) - Level 25 - Available after Herb Plant Set 1
- Insomnia 2 - Glow Wolf - Level 25 - Available after completing Insomnia 1
- Forzen Ice Cream 2 - Ice Vivi with Soft Ice Bits from them - Level 28 - Available after Frozen Ice Cream 1
- The Oppression of Ghosts - Ghosts - Level 37
- Harvest 2 - Hoes from Hobs - Level 38 - Available after completing Harvest 1
- Bone Imp Bags 3 - Bone Imp Bags from Bone Imps - Level 39 - Available after completing Bone Imp bags 2
What to do at level 40?
Those were not all the repeatable quests and you probably won't need to complete a majority of them to reach level 40. For more information on quests you can visit the Fiesta Wiki. This will also help you complete pre-requisite quests for some of the reps above. Once your baby hits level 40 it is time for the Tower of Iyzel. Just because they are level 40, however, doesn't mean that can beat the tower on their own. If you have a few friends, this could be pretty easy, but, if not, I should have an runner that can help you. Head to Roumen, visit Templar Maria and pick up all the quests that mention the tower than find a runner or get your party together and head to Forest of Mist to find the tower itself. With all the quests picked up, one run should get you to level 41 or 42. Congrats, you're not a baby anymore 🥳!