Is there a plural of this phrase that preserves the sprachgefühl?
The obvious "those are my boys" somehow doesn't feel right.
I'd feel fine saying or hearing "That's my boys!".
"That's my boy!" is literally a prideful expression of the exclaimer's relationship to a single boy which also implies some degree of personal responsibility for or shared ownership of the boy's success which prompted the exclamation.
"That's my boys!" implies the same degree of pride and personal connection as it sounds similar to the well-known singular form. However, its literal meaning is different: it's more like an abbreviation, e.g. of "That's [how] my boys [do it]!", or "That's [what] my boys [can do]!". These statements emphasize the ease and regularity with which said boys perform successfully. I find that the singular form can actually be exclaimed with this meaning in mind as well (e.g. "That's [how] my boy [does it]!").
This pluralization of the phrase has the benefit of also fitting the phonetic pattern of the singular form as it has the same number of syllables and can be exclaimed with the same sing-songy inflections in pitch.
I'd go with "Them's my boys!"
It is not "grammatical" but it preserves the casual and dialectical feel of the original, as well as the prosodic features.
You can just try : "My boys !"
If the questioner is using "That's my boy" to indicate that a certain action or behaviour, such as a comment, which has just taken place is highly characteristic and representative of the male person to whom it refers then more appropriate than "those" or "them's" would be "That's my boys".
"That's my boys" effectively says "That's typical of my boys", "That's the way my boys are", "That's how it is with my boys".
It should be noted that in this scenario the phrase may not actually refer to the speaker's child, in the same way that "Go on my son" is often used between male friends.
An example conversation where such use of the phrase would fit well might be where a woman hears from a friend that her husband and son were seen returning from a football game, cheering victoriously and waving their scarves. The thought of this scene is one that amuses and touches emotionally - and seems both familiar and unquestionable - so the women smiles and says "Ahh yes... that's my boys!"
If the intended sense of "That's my boy" is "Well done young man!", then a colloquial plural version, certainly in the UK, would be "Good lads!" The problem with "Them's my boys" as a plural version is that it only makes sense if said to someone other than the boys in question, whereas with the singular "That's my boy" is typically used as praise to the boy in question.
On the other hand, if the intended sense of "That's my boy" is "That person there is my son", then a plural is simply "Those are my boys".
I don't think any sprachgefühl is lost when you make it plural. I can't say I've heard someone exclaim "those are my boys" as much I have heard "that's my boy." I think the feeling that is usually conveyed by this expression is one of boastfullness. Is that the sprachgefühl you were going for? By the way, sprachgefühl is a very cool word--thanks for introducing it to me. I'm going to have to integrate it into my lexicon.
I think it worth mentioning that this particular idiom is paralleled, at my guess is preceeded by, "attaboy". This, in turn is from Titus Andronicus saying, "That's my boy" as a reference to his son. In that context, it would probably be more accurate to say that the plural is "that's my boys." That said, the common modern (American?) plural is either "Those're my boys!" or "Them's my [or me] boys".
Strictly speaking, though, when you move from singular to plural, the original would have been, "that's my boys" or perhaps, "there're my boys".
Again, though, in SAE, "Those're my boys!" is a perfectly acceptable plural.
If you're telling someone that the young men in question are your children, then "Those are my boys" sounds right to my Midwestern ear.
If you're telling someone that you are proud of the achievement(s) or accomplishment(s) of these young men then "That's my boys!" would sound right. If addressing the young men directly, though, I'd probably use "Good job, boys!".
"That's my boy! is an expression probably loved as much at the boy himself.
Learn it, love it and keep it in your armoury of English idioms to be used at some precious moment.
You modify these things at your peril. So the answer is "no". Alas, there is no a plural of this phrase that preserves the sprachgefühl.
-But I bet some of the smart people on Stack Exchange can come up with something you mean -but using entirely different words.
replace 'boy' -> 'boys' with a different noun and try it out.
that's my dollar! that's my dollars! those're my dollars!
for me 'those are' wins in most cases.
EDIT: hopefully this adds value:
i would like to suggest, however, that whether to use 'that' or 'those' depends on whether you're referring to the actual nouns in question, or rather a single effect caused by the nouns.
if someone had individual dollars laid out on a table, and inquired as to who owns them, i could reasonably say:
those're my dollars!
if someone were to complement me about a wise purchase or investment i made, i could reasonably say:
that's my dollars!
as i'm basically saying
that effect or event we're talking about was caused by my dollars.
in the case of actual boys, two different scenarios hopefully clarify what i'm trying to convey
someone points to a male sports team and asks their coach who they are. he says:
those're my boys
the same male sports team wins the game. in celebration, the coach says:
that's my boys
both are correct in the situation they appear in. first one is about "who are these individuals?", second is about "i am speaking about the effect [winning the game] caused by the individuals as a unit".