I've been a multi-instrumentalist for most of my 40+ years, and with violin as my first instrument, I take for granted an acute sense of relative pitch.
My seven-year-old daughter's school has recently offered basic music aptitude tests to pupils in her year, with a view to starting a limited number on violin or cello. She's interested in the violin lessons, so we sat at the piano last night and I tried to give her some idea of what she might expect in the test, getting her to clap rhythms, spot the differences in short melodies and so on. By and large she did well, but I was surprised by how much trouble she had with pitch perception.
I tested her as I remember being tested, by playing two notes in succession at the piano, and asking if the second was higher or lower than the first. She has a clear understanding of this concept, and she comfortably got the first few. I gradually moved the intervals closer together. At intervals of a fourth or greater, she found it easy. At a minor third, she was ok but slipped up a couple of times. Tones and semitones she really struggled with, although she could generally hear when there was a difference.
Naturally I wouldn't expect her to be able to identify small fractions of a semitone, but I was surprised that whole tones gave her difficulty, despite the fact that she can sing reasonably well in tune.
So, what level of pitch interval perception is typically necessary for a child to be considered to have some aptitude for violin study?