For human children I generally use halflings (strongheart halflings are closer still). PF even has
a feat for this. It also has the
Young template, but it's not really intended for PCs.
These are kind of excessive. You're not just increasing numbers, you're practically turning your Magical Girl levels into gestalt Magical Girl//X levels (particularly for the gunslinger, who qualifies with a 1-level dip at that). Conversely, you get practically nothing for levelling up in your second class.
Let me give it a try:
Busou Shoujo
You have turned your magical talents towards fighting with weapons.
Prerequisites: Costume +1, and
Magus Arcana or
Black Blade
Benefit: Your Magical Girl and Magus levels stack for determining your costume bonus, caster level with Magus spells and Arcane Pool.
Special: If you have the Device and Black Blade class features, you may choose to designate your Device as your Black Blade, even if it is not a weapon of the correct type.
Special: If you have the Mentor and Black Blade class features, they are combined into a single entity with the higher mental stats of the two, which can switch between its forms as a standard action. Your Magical Girl and Magus levels stack for determining its Bardic Knowledge modifier.
Gunslinger Girl
SHOOOOOOOOT!!
Prerequisites: Costume +1,
Gun Training
Benefit: Your Magical Girl and Gunslinger levels stack for determining your costume bonus and the effects of your Nimble and Gun Training class features. Your magical girl levels also count as gunslinger levels for the purposes of prerequisites.
EDIT:
Magical Idol
You live for the stage, and your powers earned you your first big break.
Prerequisites: Costume +1,
Famous, Inspire Competence
Benefit: While in costume your Magical Girl and Bard levels stack to determine your costume bonus and the effects of your Famous and Gather Crowd class features.