I have two balance bikes. My 5-year old used an XL Puky until recently and my 2-year old uses a Specialized Hotwalk. I know the Puky is out of your price range b/c you have to order them from Germany, but it's a great size for a 5-year old. It has hand brakes, which I know a lot of the smaller balance bikes don't have, but are essential IMO for a larger child. The Hotwalk and the smaller balance bikes that *I* have seen are too small for an older child. (I never looked at the one you linked to, though. Check the handlebar height, too. Even if the seat adjusts up, the bike will be pretty uncomfortable if the handlebars are too low.)
Regarding inseams and seat heights: My 2-year old's inseam was just slightly less than the lowest seat height of the Hotwalk when we bought it and it wasn't an issue. He would walk the bike around and didn't even want to sit on the seat for a while. Now a few months later it fits him fine, and he uses the seat as he zooms all over the place.
Honestly, if I were you, I'd go for the Trek Float for your 5-year old and plan on buying a small balance bike for your little one in a year or so. Then your older child can use the Trek bike for a few years until his sibling outgrows the little balance bike.
Tara
living a crazy life with 3 boys
I am thinking now
of grief, and of getting past it;
I feel my boots
trying to leave the ground,
I feel my heart
pumping hard. I want
to think again of dangerous and noble things.
I want to be light and frolicsome.
I want to be improbable beautiful and afraid of nothing,
as though I had wings.
~Mary Oliver