How to easily use my old iphone without a sim card?

I have a iphone 3g 4.2.1 that I don't use anymore. I want to be able to use it without a sim card to give to my kid. I've tried all the things that the internet suggest but none of it has worked or it simply is too confusing.

Operating a simcard phone without a sim card is illegal, and will not be able to receive a signal from the towers. The only number you can call without a sim card is 911.

all cell phone service providers have their towers set up to only send signals to legally purchased sim cards placed in an associated phone. That is, the serial numbers on the sim card must match the serial number on the phone in the cell phone providers computer systems.

sorry, but companies have the right to protect themselves from fraud. You might be able to get a good deal to add this other phone to your account for your kid to use.call up your provider and ask what is available. Some programs only cost an extra 5 bucks a month depending on who your provider is. You can always threaten to cancel your whole account if you can't come to an agreement, sometimes this gives the agent permission to offer you special promotions not available to the general market, just to save your business.