Dear Rabbi Simon,
If Plag haMincha is the earliest we can light candles, daven Maariv and bring in Shabbat, which is about 7.30pm currently, may those of us with young children start Shabbat dinner before then and light candles/daven during a break in the meal?
This is a perennial challenge. Some families have dinner for younger children first, and older children/adults later (ie, after returning from shul, eg ca. 8:30pm). Your suggestion is an option, but it is not ideal for adults, as one’s appetite for Shabbat dinner is diminished if you have already eaten part/most of the meal by that point. This detracts from the honour of Shabbat. Unless, of course, you do not actually eat, but just keep the children company.
In any case, strictly speaking you may [daven Mincha in advance and then] light candles, daven Maariv, recite kiddush and have lechem mishneh after 7:20pm (on Friday 27 July; it now gets earlier week by week) and then continue your meal. Depending on the age of the children, as an educational exercise you/they may want to recite kiddush and have lechem mishneh earlier, ie 6:00pm, if waiting for the real thing at 8:00pm or later (ie once you finish lighting and davening) is too late for them.
Of course this option is less than ideal, as one should preferably daven Mincha, Kabbalat Shabbat and Maariv in shul rather than as a stay-at-home experience. But this is a personal judgment to be made based on the particular circumstances of each family.
Rabbi Rashi Simon