Hian.
I don't think it's established when and even if they come back.
No, but we have no reason to think otherwise. They don't have any other headquarters. We see Tifa come out of sector 7 in a carriage while dressed up, it seems unlikely she was travelling from somewhere else.
They might not come back at all, and indeed they should be prepared for that.
The fact that they leave Marlene alone, knowing they might never be able to return for her, is indicative of bad parenting. The fact that they happen to have the good luck to return does not change that, and as such is a red herring.
This presupposes that no arrangements have been made for if they don't come back, when we have no no way of knowing that one way or the other. And Biggs, Wedge, and Jessie pull out early on the mission. We don't know where they're going, but we also don't know where they're not going. It seems most likely to me that they're going back to HQ. I could be wrong, but we're presented with no alternative.
As for possible solutions to some of the practical concerns I raised - this also entirely misses the point.
Tifa could have left food out - Marlene could also choke to death on that food as she might not be able to pace herself properly when eating, and nobody will be there to help her.
The clogging of the toilet could have a huge taxing effect on their personal economy and ability to properly feed Marlene. Hey, Barret takes money out of Marlene's schooling to pay Cloud to help him commit terror.
Some of the practical concerns that you raised only arise if left alone for longer than part of a day. For the rest, nobody at all was saying that leaving Marlene alone is a good idea. But if there's no other option, and sometimes there really isn't, the risk can be mitigated to a degree. "Keep out of reach of children" is a thing that can be done for the most dangerous items. At 4, Marlene probably isn't tall enough to reach the bar unless she has something to climb, which takes a lot of dangerous things out of the equation short of bad luck, which is indeed possible.
Also relevant is that the people living with the child are going to know its limits. If Marlene has trouble with eating, dressing, using a toilet, et cetera, Barret and Tifa will know and factor that into their plans. Any problems that result in 'making a mess' aren't that big a deal, especially since Tifa runs a bar. A mess in the bathroom/on the floor is unlikely to be new or surprising.
The point here is that, by not being here, there's a million and one ways in which Marlene can fetch herself up, which would not happen if they were there.
Entirely true. I am not and never was claiming that leaving Marlene alone is perfect parenting. My stance is just that on being left alone, once, for a short while, it is reasonable to expect her to manage to not harm herself in the interim. It is the kind of thing that parents in difficult situations sometimes have no choice but to do. They are often not happy about it, but sometimes there really is no choice.
On the other hand, I can see where you're coming from, and I don't think it's time for anyone to start namecalling. Child abuse, even in fiction, is something that can and should be taken seriously. But if expecting everyone at all times to perfectly care for children, well, a hell of a lot of fiction suddenly goes out the window. You lose practically all non dystopian YA fiction where children both have adventures and sympathetic parents, and maybe will end up not being able to read any stories with children in them whatsoever. (I'm having a slightly similar argument in another forum about Star Wars right now.)
Finally, hian, if it eases your mind, I don't have children. But you might be advised to be careful who you say things like that to, because some people could easily take a statement like that very personally, and a person having a particular opinion about a depiction of a fictional child has no real bearing on how the same person treats their own real children, especially in a few lines of a hypothetical in a gaming forum. I know what you meant, I'm not bothered by it, but I'd consider carefully the implications next time you're inclined to put forward an opinion like that. Just as a friendly stranger on the internet.
Edit: Sorry, X, wrote this before your post, if something's wrong go ahead and edit or PM me.