A book of poetry is usually welcome in the juvenile field --too bad this one is rather a weak cup of tea. Although offering pleasant sweetmeats for the sheltered child, it would seem that this gentle British number is a few degrees removed from the interests of most American children. Selections like ""The Black Fairy"" ""Saturday Cronies"" ""Nannie Says"" are too much in the hot house camp, with a sentimental rendering of an adult reaction to children. However, there are some poems, notably ""The Swing"", ""Aunt Mathilda's Cottage"" and ""The Garden Path"", which have a gay lilt and a surprise or two.