A violin shoulder rest should contour smoothly to the shoulder of the violinist. Depending on your child's body shape, a shoulder rest that cups the upper part of their back, and supports the violin well on the front, will be the best fit.
Do professional violinists use shoulder rests?
Also the choice whether or not to use them is highly personal. There are fantastic violinists playing without shoulder rest and there are fantastic violinists playing with shoulder rest. ... If there would be ONE ideal shoulder rest (or none), every single soloist would use it.