They are still a character, and more importantly a protagonist who will compare directly in the same work to 2 other (adult) protagonists. They will each need to hold their own in the reader's mind, earning their screen time.
For that reason I would not begin with her 'handicaps' as a little girl, but create a 'full' character with all the usual trimmings (wants and needs, something holding them back, an inciting incident).
Assuming all 3 interact (the other 2 are her parents maybe) it might help to create a character chart how each protagonist effects the other 2 when they are present. You may discover some new dynamics by comparing the ensemble that would not be an obvious trait for a child.
Girls vs boys (around age 6)
Again, character first because anything in this section is a huge generalization.
|compete with each other
||synchronize with each other
|constant physical activity
||constant social commentary
|compulsive: "I did it because"
||obsessive: "I know it because"
|elaborate toy car smash-ups
||elaborate doll psycho-drama
|Crying because fell out of tree
||crying because insulted
|Look what I can do (skill)
||Look at me (talent)
|winning the trophy
||getting into the best group
At age 6, the genders have more in common than different, but they are already heavily gendered in society.
Both roleplay with human avatars, but girls have 'dolls' and boys have 'action figures' – their construction strongly informs how a child is intended to interact (change their clothes or launch from an ejector seat). Girls get EasyBake™ ovens and boys get chemistry sets: similar tasks but different implied social roles. Both genders will experiment outside the box, and both are as likely to pretend to invent radio-active poisons that are fed to their stuffed animals in a macabre murder fantasy.
The gender-isms in their environment are really just set-dressing; it's valid to write her as a boy if that is easier and then surround her with a mix of appropriate (and inappropriate) toys.
Moving past gender, there will be a lot of cultural norms thrown at them, but they won't have bought into it. Every child will have 'wrong' toys, 'wrong' clothes, and a few 'wrong' tastes that are inconsequential to their personality, but basically a thing they are still young enough to get away with.
Child vs adult
At roughly 8-12, kids begin radical social development where they start to become 'aware' and can compare what they personally have to what others have – by 13 this is cemented and their social peers are far more important than family.
But rewind to age 6, they are on the high-functioning end of blissful ignorance.
Kids have weirdly lucid adult moments, but like AI there are huge gaps in logic that make you question if they understand anything or are just faking it. In real life their world is smaller, but to them it's still big and they understand it reasonably well. The Dunning-Kruger effect is strong. They don't assume there is any adult-stuff they don't yet understand, instead they will just patch-over the gaps with things they do know, and dismiss what they don't find interesting.
In adults we talk about fight or flight as a pro-active defense mechanism, but in kids it's more common to freeze – likely an evolutionary behavior that works to keep them alive. When in danger, kids will hide. It's probably the most common game they play. Kids will have a different mental map of their environment to an adult. You will have the chance to re-invent your location through this protagonist. At 6 they are starting to outgrow this, but still small enough to fit in their best places. If a 6-year-old needed to hide, they would be uniquely expert at not being found.
Try to find other ways to empower this protagonist by giving her different patterns to the adults. Give her an interior life, and her own agency and consequences that fit the genre.
"Give me a child of 7 and I will show you the man." ― Aristotle
Find the British documentary called 7 Up. It interviews 7-year-olds (in 1964) asking them basic questions about their lives and opinions. (The documentary continued to interview them every 7 years as they grew up.)
There is a section where they're asked about having a love interest. 2 boys recount a game of chasing and kissing (one enjoys when the girls scream, the other seems to prefer when the girls chase back), a group of girls are able to state which of them is the most attractive, and also which boys like which girls (apparently having already paired everyone off), a third boy dryly brags about his girlfriend in Africa and another 2 in Switzerland, a forth is worried his future wife might feed him vegetables, another boy looks shocked and refuses to answer 'those types of questions' – it is hilarious! And they are all different.
Again the take-away is that they are characters first, but at a special age where they are maybe the purest version of themselves. They are guileless little adults with all the sophisticated emotions and fallibility, already strong opinion of how things 'are' (or 'ought to be'), but with big gaps in understanding how things become that way.