What てやったら in のませてやったら mean? Is it some conditional form of のませる verb? I can't find it in dicts.
～てやったら is the conditional form of ～てやる. ～てやる is a less polite version of ～てあげる, "doing ~~ for someone's benefit". See meaning of ～てやる.
のませてやったら is the conditional form of のませてやる.
のませてやる consists of the te-form of causative verb のませる + subsidiary
And how would i translate the whole sentence: おちちでも のませてやったら どーだ?
To break it down to smaller chunks...
おちち -- breast, milk
でも* -- or something
のませてやったら -- if you let (someone) drink
どうだ -- how about...?
*For this usage of でも, see Meaning of “でも” in “食事でもどうですか？”・The use of でも and ででも in this sentence
おちちでも のませてやったら どーだ?
"How about letting (someone) drink milk or something / suck the breast or something?"
"Why don't you give (someone) milk / the breast or something?"
By the way, is どーだ equal to どう and means "how"?
Right. どーだ is どうだ, どう "how" + だ (copula).
And why だ is in the end of question sentence?
It's quite normal, especially for males, to end a wh-question with だ in casual speech, as in
「～だ？」 sounds rather masculine. Females would tend to say 「～どう？」「～なに？」「～どこ？」「～だれ？」 instead.