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][1]:

> 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.


  [1]: https://support.apple.com/en-us/HT204360