1st SW tank; 29g Biocube or 40g Breeder for baby room?

Hello everyone, I am thinking jumping into the world of SW and getting my first fish tank and wanted to put it in my 2 year old boy and 1 year old girls room but not sure what to get. I was set on the Biocube but then I read that people like the 40g breeder size and PetCo has the $1/1g sale now. I don't want anything bigger than a 40 since LR cost almost $10 a LB and not sure what sand costs and I am new to this. I am sure there will be a lot of stuff I am leaving out since I am trying to post before bed and when my kids let me on my computer.

I would like the all in one setup but I read I will need new lighting so if I have to get individual equipment will be fine too. A big question is how loud will the tank be with everything running at night while my kids sleep? They have a white noise machine so some noise is great, just not sure how loud a tank is.

Now I don't know much about Corals and their names but I know I would like some, a couple fish, crabs, snails, star fish. I have read a lot here but now it all blends together so I am not sure everything that is compatible.

Regarding live rock and dry rock(?), would it be best to get dry rock and wait till it turns to live rock so I can get used to keeping a SW tank before I put Corals in? Or is that not needed and just get LR, sand and Corals at once and just go for it?

Thanks for everyone's time.