A 10W iPad charger should in theory work and you would be correct that it would be a slow charge. Well, to be accurate, it would be a horribly slow charge - you are getting a charger that is 1/6th (17%) of what is required.

Will there be a problem? No.

Will it charge? I doubt it. I would be surprised if the MBP accepted the connection and started charging the battery from it.

A 10W iPad charger should in theory work and you would be correct that it would be a slow charge. Well, to be accurate, it would be a horribly slow charge - you are getting a charger that is 1/6th (17%) of what is required (61W).

Will there be a problem? No.

Will it charge? Slowly. Molasses in January slowly. From Apple's web site:

Your Mac will charge from USB-C power adapters not manufactured by Apple if they adhere to the USB Power Delivery specification.

Assuming you are using a genuine Apple iPad charger, it should charge since Apple charges adhere to the spec.