I highly suggest you check in with Gambler. I believe he just bought a TTR for his little one and could probably give you a few tips. I know he had a few mods in the back of his mind and they might help you along the way as your child grows into and out of the bike and acquires more skills/confidence. I was just at the local shop admiring the "orange" bikes and those little rigs are sooooooooooo nice looking. Me thinks she liked the color more than anything. And as kids always do, she ignored the hefty pricetag affixed to the bike.
As for you? I think the 450F is WAY too much bike but it's your body so thrash/abuse it anyway you want.
Keep us up to date. Always fun to watch a kid's enthusiasm for bikes grow.