Lots of practise on the bike helps to build their confidence - I helped out at a session this week teaching secondary school aged kids to cycle - some were cycling within 5 mins, whereas others took the full session length. With the older child - you could create a few games such as wiggling around a few jumpers on the ground, or see if he can lift his hand off the handlebars. With the stabilizer child - get rid of them, lower the saddle - so their feet confidently reach the ground and just get them trying to walk and occasionally lift their feet off the ground. So they aren't trying to pedal at first - if they keep hitting their ankles on the pedals you can remove them temporarily.
And report back on how they do
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Pretty much what I was going to say!
With a bit of thought, you could perhaps set up games that they can both compete in (little slalom races for example, things where taking care matters more than speed and strength. If you google gymkhana games, you'll find a few pony club things that might be adaptable for bikes.)
If you have a decent safe route nearby (all cycle path, or very quiet roads), how about a little Expedition? Pack a picnic, head out for the morning. It might only be a few miles, but it'll be a lot to them, and a ride is so much more fun with a point to it.
Quiet residential roads are a good place for practising turns and indicating and so on...