I think children of this age can be left downstairs alone for a few minutes (while you pop to the loo or are doing a quick job etc) or for a bit longer if you keep popping down to them or shouting down to them and having a conversation, but having access to the whole of downstairs alone for an extended period of time isn't a good idea if an adult doesn't know regularly what they are up to.
Quite simply a 3 year old in particular can't be fully trusted not to touch things they shouldn't and shouldn't be put in a position of being responsible for such judgement for a whole hour without adult contact.
There is a difference to being upstairs doing a a quick job,but alert to noises downstairs and being asleep.
There is also a difference to a child being left alone in their own bedroom at night and having the run of downstairs alone in the daytime. In the nighttime, much of the time is spent asleep or at least not as active in the daytime. Yes there may well be toys in the room, but other hazards which are downstairs such as knives, electricals etc are not there.
Additionally, at night, the chances of both children being up together and getting into mischief together are much lower.
The problem with the OPs idea is a hole hour of her being fast asleep and out of it, whilst the 2 children together have a whole hour of being awake and alert downstairs. The risks seem too high for that age group. Dozing on the sofa would be safer - a full scale sleep is not really viable, as an earlier poster says unless the children are also asleep.
OP also mentioned being hung over. Sounds to me like that was what created the need in the OP for more than a doze - a 6am get up in itself isn't unusual or too awful for parents with children of such ages - most parents can get through the day without needing to go back to bed when getting up at 6. Sorry to sound judgemental, but if drinking means you cannot cope with the day and provide adequate childcare for the kids, it's better not to drink. It doesn't seem right that the children should be placed in a situation where their safety could be jeopardised (however unlikely a crisis is to occur) because of a night of drinking the previous day.
Have a doze on the sofa, arrange for a friend to take the kids for an hour,go out for some fresh air, avoid being hung over.......all better approaches.