I would say the most likely reason for baby multis dying is the cold or not being able to reach the water bottle (for just weaned ones).

Multis are native to Africa therefore theyll be wanting temperatures/humidity in roughly the same region, aka they probably want a little bit of a temp boost

Check out our caresheet www.redhotroyals.co.uk/?page=multis Just the main info in terms of breeding them, not sure if you have bred them previously?