Can 您 be used as a plural form? For example, if a child tries to address to both parents at the same time: 请您放心.
Usually 您 is not pluralized with 們 like other personal pronouns. This is because etymologically 您 is a contraction of 你 and 們, and still connotes second person plural in some varieties of northern Mandarin, so some would say that adding another 們 is redundant. However, since Standard Mandarin is used in regions that don't natively use the pronoun 您, there is a tendency to treat it the same as other singular pronouns by analogy, so usage of 您們 is sometimes seen. Grammarians will still object to it as a non-standard form.
The proper way of explicitly addressing multiple persons in the second person is to attach number + 位. Thus in the child addressing both parents example, they would say 您二位. In somewhat less formal contexts, 您兩位 or 您倆 are also used.
But there are lot of native Chinese who use "您们".
Look at this video, the elegant princess said "您诸位".
In a prescriptive sense, 您们 is grammatically incorrect, according to multiple authoritative sources (which includes the Xiandai Hanyu Cidian 现代汉语词典).
Nevertheless, it is extremely commonly used in daily conversation (especially in northern China), and for sure everyone would understand you, and few would raise an eyebrow. (Southerners do not frequently use 您 in the first place, so 您们 is much less prevalent.)
So, if you are writing, or taking the HSK or the Putonghua Shuiping Ceshi, then 您们 is wrong without a doubt. However, if you just want to Chinese people, I see no problem using it.
No. 您 is the abbreviation for 你们, just like 嫑 is an abbreviation of 不要, which is a very common situation in Chinese. So it's already a plural word, you can't re-pluralize it.
Actually, It's a universal phenomena to use plural form to show respect for others. For example, in English, "you" is the plural form of "thee". People use "you" to show respect to you. As time goes by, the singular form is simply not used anymore. Maybe 你 will also be forgotten in Chinese some day.
The expression "您们" is not used in Chinese, usually we say "你们", if the child does not specify to talk to his father or mother, then he can say "你们", if he is talking to his mother, then he will say "你". The word "您" is usually used as an honorific and is used in written language. With people we know well, we say "你" and that's it.