Ok, just some generals:
[if she ~ 11-13yr old]
Try this: Engage in some sort of playful activity with close physical contact (like tickle-war about some stupid thing). Then deeply kiss her (and i mean deeply - with tongue if possible) for 5-10 seconds. Dont look serious, pretend it is all part of a game. If atmosphere is right and she isnt repulsed by the idea, you can add more sexual elemets into your play. Just make it slow, be playful, and be ready to turn it all into a joke if she freaks.
[if she ~ 7-10yr old]
DO NOT make pass at her, make it so all this will be her idea and doing. Better arouse her with innocent-looking touches durning games or some other casual situations.
For example if she sits on your lap, make her sit on one of yor leg so her groin will be ginding against it. Or carrying her around on your shoulders you can drop on your back (on something soft!), then your head will be lying between her legs, neck on her groin and hands free... There's thousuands of
possibilites, be creative!
That way she wont freak and wont tell on you because of felling that she doing something 'shameful' (especially right for introverted girls). With time (if she enjoys it) she will seek those encounters herself. Then you can add more direct stimulation. Later (around time when your hands start to get into her panties) you can disscuss things with her more openly. But dont be too serious - its all just a funny private game between you two (because of this, kissing and other 'romantic' stuff better be avoided at first).
The general idea is to make it into a enjoyable game and won't scare her by your seriousnes. No looming above her with sweaty face and heavy breath. Play with her, make her laugh, make her to be comfortable with you and to enjoy being in your company.