We picked up a second hand Ridgeback MX16 for my lad at that age for £85 in great condition.
Sold it about 5 years and his little sister later for £65.
I know Frog, Isla etc are lighter and lovely bikes, but if you just want something that isn’t supermarket bike heavy and better for little kids to ride on, then second hand Ridgebacks, Carreras, Treks, and Spesh have all been great for my two, loads on ebay to choose from, and easy to shift on with little depreciation.
The other problems with spoiling them with lovely light bikes from an early age is that they – or your wallet – are going to be in for a big shock when they grow into “proper” 26″ wheeled bikes.