Is there a word for phrases such as those that are used to calm a baby down?

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.

From Merriam-Webster.com:

cooing

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

Baby talk best encapsulates the phenomenon.

Another related term is hypocorism defined in the dictionary as

the use of forms of speech imitative of baby talk, especially by an adult.

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.