Toddlers have most of the same needs as all other sims, with the small change of social being attention. Most of the time when you send them to daycare they end up coming home with low needs. When you hire a nanny you can still control your children and ensure they are taken care of. If you have to leave, the game will give you the option to either hire a nanny or send the toddlers to daycare. Toddlers cannot be left alone without at least a teen aged sim being on the lot.
