How to space the children of a div with css?

For an unknown amount of children you could use.

#parent > * {
    margin: 30px 0;
}

This will add a top and bottom margin of 30px to all direct children of #parent.

But img is not displaying as block default, so you may use:

#parent > * {
    display: block;
    margin: 30px 0;
}

Vertical margins of block elements will be collapsed. But you will have margins at top and bottom of your parent div. To avoid that use the following code:

#parent > * {
    display: block;
    margin-top: 30px;
}

#parent > *:first-child {
    margin-top: 0px;
}

This will only add top margin and removes that top margin for the first element.

The following css will work well

div > *:not(:last-child) {
    display: block;
    margin-bottom: 30px; 
} 

> selects all elements that are direct children of the div (so you don't get weird inner spacing issues), and adds a bottom margin to all that aren't the last child, using :not(:last-child) (so you don't get a trailing space).

display: block makes sure all elements are displayed as blocks (occupying their own lines), which imgs aren't by default.