You can make a reservation for Mrs. Knott's on Mothers Day. See information below.
Traditionally, American (or perhaps just Californian) Mothers Day meals tend to be a brunch buffet with champagne served (usually an endless amount of refills per glass). Other beverages are included, such as orange juice, coffee, and tea. (Although some places that are on the low end might charge for OJ, it's usually included.) There is a variety of food in a brunch buffet including breakfast foods and lunch foods. Some have carving stations for meats and/or omelets made to order. Some also have a variety of shell fish and other fish. Often times these are restaurants that already offer a Sunday Champagne Brunch, but make the Mothers Day Brunch a bit more special.
Some families simply take mother and/or grandmother out to breakfast or lunch at the favorite restaurant, which might be a little less expensive but well appreciated. From what I have noticed, it is rare for mothers to be taken out to dinner on Mothers Day (with the exception of a thoughtful husband who finds a babysitter) and restaurants tend not to put on anything special at night for the occasional. The implications of this are that many restaurants are packed with families in the morning and the afternoon of Mothers Day. Therefore, if you want a celebratory feeling with many other mothers and families present, that is the time to eat. If you want a quieter celebration, then go to dinner (here I am thinking of dinner as served between 6 and 8 p.m.). Roy's a probably a good suggestion for that.
With regards to Mrs. Knott's:
The Mothers Day Champagne Brunch at Knott's Berry Farm requires reservations, therefore you will not be lining up and waiting an hour to get in. It is held in three locations on the Knott's Resort, with Mrs. Knott's being the only location outside of the park (I am surpised they are not using the hotel as well). Mrs. Knott's would be the nicest (although not fancy) and most historic.
Here is the press release from last year and a blog to let you know what it is like. The blog mentions the hours of the brunch are from 9 a.m. to 4 p.m. For some reason I didn't see that information on the press release. (Note that the ending time means the brunch ends. As with most - if not all brunchs and buffets - the food is put away by then.) No telling if the time will remain the same this year, but most likely.
Prices and deals are subject to change, of course. But Knott's doesn't tend to change those drastically from year to year. I didn't see a press release for this year yet. Last year's was published on May 1st. You might find information on this sooner through reading the on line newspaper OC Register's Around Disney pages.
Since the park at Knott's Berry Farm closes at 7 p.m. that night, the places in the Knott's Market Place close around that time as well. Mrs. Knott's may close an hour later, though. If you prefer dinner and want to go there, I would say get there no later than 6 p.m. just to make sure they are open. There could be a line, to get in, but I doubt it would be long at that time if they have the same special going on this year.
Here is another idea:
In you other thread you were considering reserved riverside seating for Fantasmic just for your sister and your mother. (Your sister having a recent knee injury.) If they have that, they will be served desserts and coffee by cast members. Perhaps the night for Fantasmic could be the Sunday of Mothers Day. I'm sure they will feel special treatment by the cast members. Of course, that leaves you out of the celebration. You could mix it up with a Mothers Day Brunch (either at Knott's or at Disneyland Hotel) and Fantasmic at night with special seating for them.
I do want to mention that it tooks like, if you were considering going to Knott's on your trip, Mothers Day would be a good day to visit the park - if they are going to give free admission to all mothers who reserve the brunch as they did last year.
Edited: 6:47 pm, March 28, 2013