The cheapest and easiest way to get Eras Tour tickets was last July when they first went on sale, and it was very competitive then.
Method 1 - those who had preordered Midnights got access to an exclusive pre-sale with the option to buy up 4 tickets at any of her UK & Ireland dates.
Method 2 - you had to register interest in advance for the cities you would wish to see her in. You may get selected and given a code which granted you access to buy up to 4 tickets at a specific city. Many people did not got codes at all, within my group we registered 21 times across various cities and only got 2 codes. These codes (AFAIK) are still live and can be used to buy further tickets which are put on for sale. Getting a code in no way meant you were getting tickets, it was a battle.
Method 3 - official resale (via TM or AXS), a code is not needed and there are price caps in place so the best option if you don't have a code. Guarantee you are not being scammed or hugely overcharged, they are like gold dust and get snapped up quickly as demand for the show is so high. It's luck of checking at the right time.
Method 4 - unofficial resale (viagogo, stub hub...) these are much more risky with scammers operating on them, and the price inflation is massive. Many people have bought tickets on there and had no issues, but you can't be sure you would not be unlucky.
As a side note, I agree with PP, while it isn't inappropriate for children, it is not designed with them in mind. The show is long, there is plenty of swearing, quite a sexy dance, and things that will go over their heads. The kids around me were worn out and sitting down, some even curled up sleeping, by 1 hour left, and given everyone is standing and dancing the whole show when seated they wouldn't have been able to see anything. A little girl by me also found one particular song a little frightening, another kid had to leave after 10 minutes as it was too loud and intense for her.