In URLs, special characters are usually replaced with percent encoding. Does it work for you when replacing the & by %26? So, in the case of Jack&Jill, it would look like Jack%26Jill. Not sure if this works for your specific use case since you haven't really specified. If this doesn't solve the issue, could you include some screenshots of what you're trying to do in your original post?
You can use urlEncode
Are you passing the values into or out of the Mendix application? As others have said & is used as the separator for parameters, so everything after a & character is seen as a new parameter.
If you are passing the values into the Mendix application, it’s up to your calling application to properly encode the URL.
If you are passing the values out of Mendix to another system, then you must escape the values. If you are building the URL as a String, look at using the urlEncode function to encode the individual values, but not the URL as a whole.
Hope this helps.