I have a question and promised my 7-year-old daughter that I would check with a Rabbi. My 9-year-old son has been learning to say the Friday night Kiddush. It has been a very very slow process and he has found it very difficult. I am extremely proud of his persistence and effort he has put in and told him he can say the Kiddush at Shabbos tonight. His younger sister is the opposite, she finds reading Hebrew very easy and demanded that she be allowed to say the Kiddush as well. I told her girls aren't allowed to do it. She was horrified and told me to check with the Rabbi if that is true. I feel like I've handled this badly and opened a can of worms. Are girls allowed to do say the Kiddush? Are kids allowed to say the Kiddush?
You should indeed be proud of your son's hard work and achievement, as well as your daughter's talent and enthusiasm. You should be even more proud that this is an issue for your kids - may you always have such problems!
Now let's get technical. Kiddush is a mitzvah that both men and women are obligated in once they turn bar/bat mitzvah. One person can recite it, and those who hear it and say 'amen' have done the mitzvah too.
When a man over bar mitzvah is present, he makes Kiddush. When no man is present, a woman over bat mitzvah makes Kiddush. A child can make Kiddush, but not on behalf of an adult. A child is not yet obligated in mitzvos, so cannot perform a mitzvah on behalf of someone who is.
So in your case, your son can make Kiddush for himself, but not instead of you. As an adult, you should make the official family Kiddush.
Now in theory your daughter can make Kiddush for herself too. But I would encourage her not to do that. Rather she should light one Shabbos candle each week and say the bracha.
Both Kiddush and candle lighting serve the same purpose, to acknowledge the sanctity of the day. Kiddush is a masculine way of doing it, making a verbal statement proclaiming the holiness of Shabbos. Lighting candles is a feminine way of doing it, creating an atmosphere and aura of holiness.
One day, with G-d's help, your son will be the father and make Kiddush for his family, and your daughter will be a mother and light the candles for hers. The idea of education is to prepare each child for those obligations that will apply to them when they are older.
But there's another reason to gently steer your daughter away from making Kiddush. She may upstage your son. When his younger sister recites with ease what he worked so hard to do, he may feel belittled. Better they each flourish in their own distinct ways without any competition or comparison.
You didn't open a can of worms, you have opened the opportunity for a great teaching moment. Not only can you guide your children in observing mitzvos, you can teach them an important lesson in sensitivity too.