Van Dyck has painted the clothes in more detail.
A century later Reynolds has learnt to paint with dash, though not with the mastery of Velasquez.

The effect of the cloak of the little Duke, its shimmering shades of mauve and pink, is inimitable.

It tones beautifully with the background, varying from dull green to brightest yellow.

The background happens to be sky, but it might as well have been a curtain, as long as its bit of colour so set off the clothes of the little Duke.
When Reynolds painted children he delighted in making them act parts.
Even in the 'Age of Innocence' the little girl is looking how very very innocent.

He painted one picture of a small boy, Master Crewe, dressed to look like Henry VIII.