As far as your silly experiment, your problem arises in that ly is used to convert an adjective into an adverb, with the definition "in a [adjective] manner".
Thus, sillily is a word ("in a silly manner")
As far as "in a sillier manner", there are two rules for forming a superlative from an adverb. If the adverb was formed by adding ly to an adjective, you must use most or more.
"In a sillier manner" -> more sillily
"In the silliest manner" -> most sillily
If an adverb is the same as the adjective form, you can make a superlative using er or est
"In a faster manner" -> faster (ex. "He ran faster")
"In the fastest manner" -> fastest (ex. "He ran fastest")
As for sillilily, this is entirely nonsense. The reason is that our ly rule only applies to adjectives. You can not apply ly to an adverb and expect a "double adverb". An attempt to apply the rule directly would result in:
"In a sillily manner"
Notice that since manner is a noun, it should not have an adverb describing it. In your examples the word "suggesting" came out of nowhere. Nothing in the grammatical construction implied that there was suggestion.
In the case of sillililiest we encounter both of the above problems simultaneously. First, you attempted to make an adverb from an adverb by adding ly
sillily -> sillilily
Then you attempted to apply est or er to make a superlative
sillilily -> sillililiest
Both of these can not be done. The first because ly only applies to adjectives, not adverbs. The second because to make a superlative from an adverb which was formed by adding ly, you must use 'more' or 'most'. Again- you introduced the word "suggesting" which came out of nowhere.
The two children are members of a group and the comparative or superlative relates to that group.
Expressions like the ten best universities or the cleverer children in the class are quite common and I don't think require further explanation.
As for her two younger children, both forms are grammatically acceptable but I prefer her two youngest children because it is clear that the superlative implies comparison to the entire set. Use of the comparative, on the other hand, brings some confusion, because there is some ambiguity as to whether there exists a younger child than the two present, although I would probably assume this is not the case. We also have to disregard a potential comparison with the woman herself, which would be a pleonasm.
Best Answer
In English, the idiom is: [subject] is no more [noun or adjective] than I am is very common. Please check out my examples:
He's no more a thief than I am.
He's no more rich than I am.
He's no more young than I am.
Those sentences above are idiomatically sound.
This idiom should not be confused with:
He's no richer than I am. He's no younger than I am.
In the first group: no more + adjective is not a comparative adjective. It is just a regular adjective. He is rich. He is not rich. He is no more rich than I am rich.
Please: "He's no more young than I am" implies we are both old. Just as: "He's no more rich that I am" implies we are both somewhat impecunious.
No more here means: His condition or state is not rich just as mine is not. Not at all rich. I'll leave poking at the grammar here to others. Frankly, I can't be bothered. :)