A few ideas for younger faces...

1. Soften. You mention "rounder head" but this must go beyond general head shape... soften the cheeks, eliminate/reduce angularities, especially in jaw area.

2. Pay attention to the nose. The nose tends to point further downward and have the bridge come out with age... so push the nose tip upward a bit, and recess the bridge.

3. Try making the lips a bit fuller.

But it's probably most important to look at photographs. Find a photo of a child or teen that has a general resemblance to the character you're working on. Then compare what's different. It's very easy to overlook specific features when dealing in the abstract.

______________

Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3