are girls allowed to eat before kiddush on shabbas if they did not daven yet?
There is no difference in principle between boys and girls with regard to eating before kiddush. Like boys, however, the obligation to make kiddush, and the corresponding prohibition to eat before kiddush, is only incumbent after davening. For girls, the obligation (and the prohibition to eat before kiddush) will come into place after she davens the way that she usually davens: for a girl who davens shmoneh esrei, the prohibition to eat before kiddush will only apply after shemoneh esrei is davened; for a girl who davens less, the prohibition begins after she davens the way she is used to davening.
It should be noted that this answer has not dealt with the separate prohibition of eating before davening, which also applies to women, but regarding which there is room for leniency if it will be difficult to make kiddush after davening.
Sources: Mishnah Berurah 289:6; Shemiras Shabbos Kehilchasah vol II, p. 151:152.