How to say "Lac Puerorum, or Mylke for Chyldren" in modern English?

Without context it's a bit tricky. It seems that the phrases you are having trouble with are either Latin phrases or English phrasing with archaic spelling. 'Mylke for chyldren' phonetically works out as 'milk for children,' which today would more likely be expressed as children's milk.

It's not as simple as that though, and this is where context comes in. Are we talking about children who are drinking a glass of milk, (usually cow's milk) or are we talking about a baby? In the latter case we would use the term 'breast milk.'

It's the title of an old book. he's got hold of something like a list of antiquarian books, and he thinks he can just translate all the words in the titles into modern English.

The Latin would be left alone, so the title in modern English would be

Lac Puerorum, or Milk for Children

Lac Puerorum means milk for children. I believe we've had this question before. This is a Latin grammar for English-speaking children, and was published in Antwerp in 1511.