Player safety needs to be addressed immediately regardless of gender. This should be rule #1 of any official or parent on the sideline - period.Here is what you should do, whether you are a coach or a parent. Shut your mouth and let the game go on. None of your real objections have to do with "player safety." Every game played at ages 9-12 has a few players who are much bigger, or much more physical, or further ahead in terms of puberty. Nobody tries to disqualify bigger kids based on "player safety," otherwise there would be weight or height limitations attached to each age group. That 10-year-old girl who is 6 inches taller and 30 pounds heavier can muscle out all of the other players on the field with impunity as long as she follows the rules of the game. And afterward, you and your coach will be in the parking lot trying to recruit her. (I would.)
Your claim that having a 10-year-old transgender boy creates a "safety issue" is a transparent pretext for your true agenda. You don't like transgender kids, and you don't want one playing against your daughter.
Let me ask you something. Suppose that a 10-year-old "boy" truly identifies as a "girl" and wants to play soccer with other girls. Don't you think that it is hard enough on her without parents like you posting and publicly objecting to her right to play?
A little more compassion on your part might be in order.
You don't want specifics mentioned about a 10 year old player but then want to generalize "none of your real objections have to do with player safety" and "you don't like transgender kids and you don't want one playing against your daughter". Take a look at this thread and you can see that several have jumped to the assumption that anyone who thinks that this is worth even having a conversation about is "anti-trans" and you can see where the real issue lies.