The premise is the iPad is stolen; even if the authentication code is shown it has to be entered in the originating screen and for that you require the id and password.
Concern is this - If OP goes to Apple's website and logs into his/her/etc.'s account, Apple (the way OP has it setup) will send a text to his phone, which will be relayed to his iPad. Then, if the iPad is stolen, anyone can use the iPhone or the iPad to request access to the iCloud account, assuming they already have OP's iCloud password.
Frankly, the whole thing seems pretty far fetched.
(1) Need to know OP's account e-mail
(2) Need to know OP's account password
(3) Need to have physical access to OP's phone or tablet
Pretty much at this point OP has no one to blame but themselves. For all of this to have happened OP would have had to basically employ no common sense protections. Two factor is something you have and something you know - if you give out the "something you know" and you lose the "something you have" then they aren't appropriate for use. Basically, OP needs to protect their information and devices better.