Tbh neither of them need milk in the middle of the night, I would just offer plain water.. they will soon get bored and hopefully get the message that there is no milk on offer in he middle of the night.
As for your eldest, my LO went through the exact same thing, took to the bed straight away but became unsettled in the weeks to follow.
we just stayed consistent and would pop him straight back to bed, no convo, fuss etc just straight back to bed. it took a fair few nights for it to sink in but we got there
He still used to try it on and get out of bed but we'd just shout up from downstairs 'bedtime, good night!' then he'd just say nite nite and off he went. Could be worth a try. like i say it took a fair few days but we got there..just remain consistent and don't give into any demands or request, once you do your making it 10x harder for yourself and sending them mixed messages.
