Hi Mandyltolbert 12,
I’m not certain why you added the topper (it probably wasn’t necessary) but either way congratulations on your new mattress 
You certainly made a great quality/value choice.
There is more about the pros and cons of different types of mattress protectors and some examples of each of them in post #89 here. With a 3 year old I would make sure that the protector you choose can protect against any “accidents” that are likely to happen.
I would use the protector over the topper.
Phoenix