Because some family members are gluten-intolerant, we started baking two kinds of cakes, etc. that look and taste almost identical. Are the "Shehakol" pastries acceptable for the "Mezonot" foods that usually follow Kiddush? Answer:
According to the consensus of poskim and many (not all, and it may depend on the type of sensitivity) health experts, oats can be used as a wheat substitute for gluten-sensitive people, and the halachot are identical to those regarding the other major grains. While you look into the health feasibility, this answer is written for one who cannot eat anything that is Mezonot.
The gemara (Pesachim 101a) says that Kiddush must be done in the place where "a meal" will follow. On the other hand, it refers to "tasting" after Kiddush, implying that a full meal is unnecessary. While some say that this taste must include bread, the Shulchan Aruch (Orach Chayim 273:5) rules like the Geonim that wine can also be used. While there are opinions in either direction, the consensus is that a revi’it (approximately 3 fl. oz.) is needed and sufficient (see Mishna Berura 273:22, 27).
The Magen Avraham (273:11) reasons that if the Geonim can accept wine in this context, then it is true of anything made from the grains, which is more meal-like than wine (see Shulchan Aruch, OC 291:5 regarding the laws of seuda shlishit). This is the source of the common practice of having cake for the post-Kiddush "meal." Along these lines, the Pri Megadim (Eshel Avraham 273:11) permits dates for this purpose, based on the halacha that if one mistakenly made Birkat Hamazon on dates, it is valid after the fact, because dates are particularly filling (Shulchan Aruch, OC 208:17). The Tosefet Shabbat argues, and most Acharonim say that one should not normally rely on dates for eating after Kiddush (see Kaf Hachayim, OC 273:42; Yabia Omer VII, OC 63).
While according to the spirit of the law, there may seem to be little difference between the Shehakol and Mezonot pastries, the former do not fit the halachic parameters for the eating after Kiddush. As we have seen, there are other alternatives. For cases where there are not viable alternatives, we mention two fringe leniencies. There is an opinion that in a case of need, any food can be used (Chayei Adam 6:22). There is also an opinion that Kiddush works for all assembled if one person eats the requisite amount (see B’tzel Hachochma IV:2). If one must rely on one of those opinions, it is proper for him to eat something that fulfills the spirit of the law.
We will now discuss a related context where the spirit of the law is important (and might fit the letter of the law also) – the full meal. One needs to have two loaves of bread/challa, eat a k’zayit, and recite Birkat Hamazon at the end of the meal, and these require halachic grain. It would be regrettable for a gluten-sensitive person to consider himself as incapable of fulfilling the mitzva of seuda, prompting some to do whatever they feel like. It is proper (we cannot create an outright obligation) to have two nice loaves of bread, of whatever flour one can use. There actually is halachic precedent of bread that is not subject to Hamotzi and Birkat Hamazon. The halacha is that an eiruv chatzeirot must consist of bread, but the bread can be from rice or lentils (Shulchan Aruch, OC 366:8), because that is considered legitimate bread (Mishna Berura ad loc. 47). Notice also that the concept of loaves of bread is learned from the manna in the desert, and that was not made from normal grain. What is important is that this was their bread (see overlapping idea in Minchat Yitzchak III:113). For the gluten-intolerant, these are their breads. While we would not suggest such an approach if one has the opportunity to follow the regular rules (including using oat challa), one who is in such a situation should view his meal as a seudat Shabbat.
It is also worthwhile to drink enough wine, or eat dates or another relevant food, to enable the recitation of a long Beracha Acharona, which contains the basics of Birkat Hamazon and mentions Shabbat.