Speakers of Purik-Tibetan may say "kangchu Tululululuuuuu" in order to comfort a baby they're cradling in their arms. To someone else, they might also say that they are "doing the Tululu". What's a good (and perhaps very general) word to refer to such a "phrase"? Unlike a lullaby, it's not sung (even though the repeated lu-syllables sound somewhat chanted), and it doesn't make any sense (at least not from a synchronic perspective).
A speaker of English might say "kangchu Tululululuuuuu" to an infant child, as well. This type of nonsensical vocalization to a child is commonly known as baby talk.
Baby talk, also referred to as caretaker speech, infant-directed speech (IDS) or child-directed speech (CDS) and informally as "motherese", "parentese", "mommy talk", or "daddy talk" is a nonstandard form of speech used by adults in talking to toddlers and infants.
It is usually delivered with a "cooing" pattern of intonation different from that of normal adult speech: high in pitch, with many glissando variations that are more pronounced than those of normal speech.
Different people have different catch-phrases for how they speak or make noises at a baby. A common one (which strikes me as oddly similar to your example of "knagchu tululu") is some variant of cootchie-cootchie-coo.
Baby talk is a more general term than cooing. Cooing does have a direct connotation of a soothing purpose, while baby talk encompasses multiple purposes or motives (and thus, a +1 to you @THEAO); the most common purpose of baby talk is (in my limited, non-parental experience) is entertainment for the child or the talker. Cootchie-cootchie-coo is associated with tickling and laughter. Also, it is not uncommon to encounter people in the USA making noises (and/or faces) at seemingly totally disinterested babies.
As a final note, depending on the setting, you may also find parents shushing their children, infants included. This is more common in public locations where a noisy child is disturbing to others.
I would refer to the action itself as soothing a child.
And in verb form, I would say "cooing".
The mother cooed at the fussy baby.
1: to make the low soft cry of a dove or pigeon or a similar sound
2: to talk in a soft, quiet, and loving way
There's also the word coddle, which directly means 'to treat as a baby' but also means to soothe said baby in such a way.