While not wanting to be the bearer of bad news, here goes...
A wooden bow that has developed set does so for predominately two reasons: 1) The belly wood has been excessively compressed, and/or 2) the back wood has been excessively stretched. There's a number of scenarios that can cause this, but once it happens, it can't truly be reversed. Offset, but not reversed.
Heat can be used to induce reflex and temper the belly, but it only offsets it. In your case, you can't apply heat because it will most likely cause the glue to give, leading to a delamination. Sinew will just impart even greater stress on the belly. And you never want to bend a wooden bow backwards. The only thing it will achieve is a momentary adjustment suitable for a good picture. It can actually cause the problem to get worse and/or break the bow altogether.
Since it's a light child's bow, your daughter will probably outgrow it soon anyway. You can possibly add a belly lamination, which I've done with success in a few cases just such as this. But the work involved is not always worth it. As a final suggestion, the bottom limb almost always receives more stress and, in the case of wooden bows, can take a more pronounced degree of set over time. Flipping the bow over (if possible) and making the bottom limb the top limb can sometimes improve the situation slightly.
Anyway, I hope my $0.02 helps!