These are what I think how the brothers would be if they were single teen parents with a human child toddler
(Technically they are teen's right? If they are seniors)

Oh and I'm not going to make these scenarios all fluffy there is a lot of angst as with every family
But remember the brothers are demons
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Lucifer
•He would keep you to his side at all times that he can
•VERY PROTECTIVE
•He would refuse to let you go. But with Lucifers work and position of head of his family he has to so tends to give you to one of his brothers to watch you
•Lucifer really wouldn't play with you or do anything childish but have his brothers instead I feel like it would hurt his pride deep down but on the inside but is to prideful to admit it
•But with Lucifer not playing with you just because of his pride starts to effect you in a long term way. With him not realizing
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When you first see your father transform into his demon form it honestly terrifies you to the core. Without him showing many emotions like smiling, being childish, or him even playing with you. Once Lucifer calms down and returns to see you again with Mammon and Asmo tending to you once you set eyes on him you start screaming and crying thinking your father is a monster petrify  of him.
It honestly breaks Lucifers heart that he sheds a tear he stars at you just staring blankly not knowing what to do. Mammon instantly starts being responsible looks at Asmo nods and looks back taking Lucifer to the side to convince him that everything will work out and be okay
While Asmo calms you down petting you hair saying "shhh baby I know, your father was just upset. Sometime it happens, it happens to all of us. He isn't a monster. Your father is a very caring man. He loves you we all do. I promise pinky promise "

Mammon
•Mammon is a toddler to begin with.
•when ever he's with you he is always has one of his brothers with him considering that Mammon would do something stupid causing your death
•Now that Mammon has you he try's really hard to keep in mind when gambling that there should be a limit always trying not to be in debt at the end of the night but usually fails in the end
• When Mammon is in debt caused stress on the both of you. Seeing your dad so upset without knowing why and not explaining what's wrong causes you to believe that you were his problem
• Lucifer often scolds in his demon form yelling at Mammon in his private library so you won't see them arguing saying that he should stop or at least think about his now child.
•With every night Lucifer yelling causes a lot of stress on Mammon.
•Mammon really want you to have a bright future,and a happy childhood. Being greed he thinks that the more you have the more happy you are causing him to gamble even more and make more promises to witches swirling into more and more debt
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One night Mammon carrying you hip to hip to get a late night snack from the kitchen sighing setting you down grabbing some fruit for you when Belphie, Beel, and Lucifer come into the kitchen
"Mammon we need to talk"
With Lucifer not quite knowing your presence you hide wanting to know what's going on
Belphegor and Beel just shrug it off knowing what's happening and instead grabbing ingredients for there soup
" Not tonight" Mammon try's to sigh him off
" Mammon" Lucifer says harsher "You must not have heard me. I said We.Need.To.Talk"
Silence
" I said not now" Harsher imitating a Lucifers tone
"Excuse me"
"You heard me" Mammon says spitting poison looking directly into his brothers eyes "Mammon you need to start thin-" Lucifer is cut off by a puff of smoke"You really think that I'm not trying don't you" That's were you looked up to see instead of the happy go lucky father your used to instead a monster in his spot at any moment ready to pounce. Shaking under the table not knowing what to do you run out of your hiding spot to hug Beelzebubs leg sobbing your eyes out. Shocking everyone they look at you as you try to hide your self behind Beel from the monster that you were now looking at.
"Shoo" Belphie says as he waves his shakes his hand at both of the older brothers "Mammon calm down then comeback" He says. Mammon is hesitant he looks at you. The terrified little human looking at him shaking clearly scared. Heart broken he leaves to calm himself down.
Bending down Beel and Belphie get on there knees "Shh...don't worry"Belphie coos an attempt to get you to calm down

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I didn't think that I would write this much oop and I originally wasn't going to have any angst but oh well

I'm working on right now the other characters I've been today for some reason wanting to write so yeah Lolol
Sorry if it's kinda confusing at parts it was supposed to be just points but I turned it into a small one shot oh well