Why accepting we exist must not only mean that God exists but that except for Islam & Judaism every major religion's concept of God must be false

I'm in grade 5 and this makes no sense version (please improve anything if you can. Also anyone is welcome to copy/paste/improve/use the original post. It's not my argument):
We exist!
What are the types of existence? Well, we know you can not have a square that is a circle. That's impossible!

Can you have a square that is red? Yes, that is possible! Does a square have to be red? No, there are many options! Red is just one of them. It is possible for it to be red or blue! My favourite colour is red and yours is blue!

Is it possible for a square to have more or less than 4 sides? No a square must only have 4 sides, silly! It is possible for a square to be multiple colours but there's only one answer for the number of sides. We said something is possible if there is more than 1 answer. Let's call it necessary when there's only one answer.

/*points to tennis ball*/ : I like that yellow tennis ball! If I cut it in 10 pieces would it be the same? No! So the tennis ball couldn't have existed if they didn't put it together with the stuff they used! The ball could have also been red! That means the way it is, is one possibility! A possible existence! It depends on that stuff they used! Its parts!
A possible existence depends on its parts all put together and doesn't have to exist! Everything in this world is made of parts including us humans!
You can't just have things that are possible! (Don't have an analogy for kids). You need something that is necessary!
Necessary can only be one, and if it had the description of a possible existence, it wouldn't be necessary! A necessary existence is always only be one, because if there is more than one answer it means it's not necessary and something else has to be! A necessary existence cannot depend on something else, so it always existed since nothing can create it and always will exist because it cannot change or there will be more than 1 answer! It also cannot be made of stuff! Like a jacket or a human which can be taken apart and changed/put together!

/r/islam Thread