Even a three year old can easily understand the significance of signing something. Especially if you, as an adult, place importance on the ritual.
Asking toddlers to promise things, to explicitly agree to future events, is a big part of preventing tantrums. Three year olds are very capable negotiators.
Yes, we adults really place a lot of importance in the ritual of "scroll to the bottom and click 'Agree'".
(In case my point is missed in the snark, it's that adults will often sign whatever they're told to sign, especially if worded like "sign this or else you can't [X]", and kids will sign whatever they're told to sign by their parents. If parents were actually placing importance on only signing things that were necessary, there's no way they should let their 4-year-old sign anything that could remotely be considered important.)
But I just wanted to make the point that four year old — or even a three year old — can totally understand the concept of signing something and a parent could even use it to enforce a future event (e.g., sign this bath time contract which means you must have a bath after dinner).
