why this scene?

One of the aspects of the film is the relation between the future and the past. Nader wants to save the old traditions (The dad) and Simin concerns about the future (Her daughter). Termeh and Somayeh(Razieh's daughter) are the future, the next generations and the dad is a symbol of the past generation and the old traditions.
This scene shows that how the future generation (Razieh's daughter) have doubts in keeping alive the old traditions (The past generation-the dad). She doesn't know that she wants him to be alive or not. She's the next generation and the old man is the old traditions.
When Somayeh and Razieh come to Nader's house the first day, Somayeh asks her mother (Pointing to the oxygen tank): "What is this?" and this show her curiosity about the oxygen tank and then later we see her playing with it. But also notice that after she asks her mother, a few moments later the dad also asks the same question. He wants to know what is that thing. And this is the relation between Somayeh and the dad. (The future and the past) They both have the same question. And also notice in one scene we see Somayeh sitting on the dad's wheelchair and playing with it. This is another sign of the relation.

This scene got a big laugh in the theater. It may be intended as comic relief to break up the gloominess of the situation (and make the eventual disaster more poignant by contrast). And it's also a kind of foreshadowing: the child's playful mishandling of the oxygen tank mirrors the more serious problems that will be occurring later.

I guess what Kasra Karbasi said is true.
But tbh I thought it's just something to show a natural re act children love to mess up stuff.

I believe the scene was intended to make Razieh a less sympathetic character early on. A mother not watching her child who could potentially kill someone with the twist of a dial. This perhaps to contrast her redemption near the end.

For me this film is an expression of the bourgeois fear of the working class. Whereas the negative traits of Razieh and her husband (resentment, perceived theft, general untrustworthiness and scheming, violence) could be interpreted as being the result of social circumstances (poverty, lack of education, etc.), the scene of the young daughter messing with the oxygen supply is meant to imply that the threat the working class poses to the middle class is of a more intrinsic nature.

This is an interesting scene, I like the idea one poster had about it depicting the old versus the new. The old must give in to the new and the old are at the new's mercy. Comic relief was another idea but I thought perhaps it was to show that the senior, although afflicted with Alzheimer's, could still be very aware of his surroundings and what was going on. He seemed alarmed at what the child was doing and was aware enough to be able to portray some humour about it in the way he smiled and engaged with her a bit through his eye contact and facial expressions. Or maybe it was just fear and he was trying to distract her from doing it again. But he was at his most aware while doing it. Was it to show that despite his affliction he didn't want to die? We see that he is alarmed at the idea that she could kill him with her actions. Maybe it was to make him seem more human to us. To portray that many afflicted people still have a strong instinct or desire to live. A few other times I saw a real sadness and understanding in his facial expressions that showed he had a deeper understanding of what was going on than we first may have thought. (That was one of many things I loved about this movie, the facial expressions, the incredible acting that let you forget it was acting, even from the teenager and child.). Another reason for the scene may have been to show the power youth has over the aged. He was completely at this child's mercy while she was unwatched and fooling around with the valve. The loss of power the aged must eventually accept and give up to the young. And that the young can seem almost heartless and unaware of their strength, health, and power over the weak and aged. (Also depicted by the child playing with the wheelchair.)

When you get up in the morning, how do you decide what shade of black to wear? (Shallow Grave)

in a more pragmatic sense, in terms of the story, i think this scene was there because that's how the grandfather wakes up. the little girl plays with the oxygen tank, waking him up during his normal nap time (it is shown on screen that he wakes up because of this). her mother, having no knowledge of this, is cleaning her daughter's shirt which she dirtied with the trash, which, during this time the grandfather manages to make it outside and go to the newspaper stand.

sry i see this post is super old. don't mean to be all reviving it and stuff.