Why not use padding with negative margins? Something like this:
<div class="parent">
<div class="child1">
</div>
<div class="child2">
</div>
</div>
And then
.parent {
padding-top: 1em;
}
.child1 {
margin-top: -1em;
height: 1em;
}
.child2 {
margin-top: 0;
height: 100%;
}