One may not eat within 30 minutes of the time of krias shema. Thus, one who makes early Shabbos has to be careful to start the seuda before 30 minutes before tzeis. For this purpose, what is starting the seuda? Is it sufficient to make Kiddush? Wash? Maybe just pour the wine in the becher?
We can not start the meal from 1/2 hour before the time that we are to say kriyas shema. The Shulchan Aruch based on the gemora, says that the start of the meal (during the week) is when they wash thier hands. Although they are not actually eating then, but this is the process done when starting to eat. The poskim therefore say that on shabbos the beginning of the meal starts with kiddush, (especially because of kiddush bmakom seuda, and there should’t be a hefsek between kiddush and washing) therefore if kiddush was said prior to 1/2 an hour before the time to say kriyas shema, they can continue, wash, and eat.
As a side point this 1/2 can even be a few minutes before shliya. For example in E. Yisroel, where many people daven maariv 22-25 minutes after shliya, a 1/2 before shliya would be 5-8 minutes before shliya, however in the states it would be after shliya, based on your minhag.
Shulchan Aruch 232-2, M:B ibid 20, Poskim.