How to show associated fields if checkbox is checked in customize widget screen using wp_customize?

This is possible with JavaScript. You have to add an event listener to the checkbox and then show/hide the inputs depending on the checked state. Here is an example:

checkbx = document.getElementById("checkbox-id")
input1 = document.getElementById("input1-id")
input2 = document.getElementById("input2-id")
checkbx.addEventListener("change", () => {
if(checkbx.checked){
    input1.style.display = "block";
    input2.style.display = "block";
}else{
    input1.style.display = "none";
    input2.style.display = "none";
}
 })

Sorry for bad formatting, I’m on my phone.