Howa makes (or at least they used to) a 2 in 1 rifle. It comes with both a youth and adult stock. You could also look into a standard Remington 700, then just buy a youth stock for around $70….use the youth stock now and you’ll still have the standard stock when they grow into it.
Edit: sorry, I missed that you said you need a left handed gun. I don’t believe they make the 700 in left handed models.