What the evidence supports, and where it is still an argument
The blue light case is the one you have heard most and the one to handle most carefully. There is laboratory work showing evening light can shift melatonin. There is also real disagreement about how much that matters at phone brightness, at arm's length, for fifteen minutes. Anyone telling you it is settled is selling something, and that includes us. The duller point is the more useful one. Screens push bedtime later, not through the light but through the clock. One more episode, one more level, one more video that started on its own. If sleep is going missing in your house, the delay is the more obvious suspect.
A child alone with a phone and a child on a call are not the same object
The solo screen has no ending built into it. The feed refills. Nothing in the design of it wants the child to stop, and a seven year old is not equipped to be the only brake in the system. A call has somebody else's bedtime in it. Pop is up at half past four to be through the boom gate by six, so the call finishes at quarter past seven whether anybody wants it to or not. That outside limit is the structural difference. It is not virtue. Somebody else has to go to bed too.
Put the screen in the middle of the evening, not at the end of it
What a child gets used to is the order of things, and the order can stay the same whoever is reading. Teeth, pyjamas, curtains shut, one lamp on low, then the thing that happens last. A call at quarter past six, then teeth, then the lamp, is a different night from a call at half past seven with the light already off and everybody wound up. Move the screen up the order and most of the argument gets quieter. It costs nothing and it survives the nights the internet drops out on the drive home.
Say the ending out loud before the call starts
An ending that turns up without warning is the one that gets argued with, so name it first, in ordinary words. "Two books tonight. Then Mum says goodnight, then I do the light." Say it before the call connects, not while it is running. Tell the adult on the other end too. They cannot see the clock in your kitchen or the mood in the room. Give them the sentence: "I'm going to say goodnight at the end of this page." A grandparent in Adelaide who knows the plan will land it. One who does not will keep going, because of course they will.
The end of the call is the hard part
The fight is almost never about the book. It is the handover from one state to another, the same reason the school pick-up line goes badly at ten past three. Three things help. Let the person on the call end it, so the child in the room is not watching a parent take something away. Send the phone out of the room, face down on the bench, not on the bedside table where it gets picked up again at half past eight. And name the next one: "Next is Thursday after school." A specific next time gives a child something to hold. A long goodbye just gives them a longer ending to argue with.
Small settings, none of them longer than a minute
Brightness right down, and let the lamp do the seeing. A phone at full brightness in a dark room is unpleasant for adults too, we just stopped noticing. Prop the phone against a stack of books at the end of the bed rather than handing it over, so the child points at the picture instead of touching the glass. Put Do Not Disturb on, so a work message does not land in the middle of page four. Turn the volume down rather than up. And keep the same time each night where you can. Quarter to seven on the nights it works beats whenever we get a chance.
What we are not going to tell you about our own app
We are not going to tell you StoryLink is good screen time. There is no such category, and if there were, we would be the last people you should hear it from. The plain description is this. A call with a person in it, at a set time, that ends. You both see the same page, one of you turns it and it turns for the other, and you see each other's faces. Whether that earns a place in your evening is yours to decide. One thing worth knowing either way. Recording yourself reading a book, to be played back later, is free and needs no call at all. On the nights a live screen is the wrong idea, or the reception is hopeless, a recorded voice with the phone face down on the shelf is a much smaller thing to have in the room.