Actually the word "Szeretlek", such as every other word in the Hungarian grammar system can be used on various and dozens of ways as it's always depending from the situation itself.
I love you = Én szeretlek téged (1:1 translation, I = Én, Love = Szeret, You = Te. Now in the Hungarian grammar different situations are giving different endings to the words, which is making every word, every situation easier to understand, even without descriptions (In English you must describe everything with I, You, He/she/it, this and that, etc, etc...). In Hungarian the words are changing, transforming for every possible situation. This is making Hungarian a bit more advanced language then most of the others and the primary reason why this is the one of the very few languages where we don't have to call people on their names so many times in dialogues (Unlike in English where you're calling everyone on their name rapidly in a conversation, or you must refering at the other person continuously as "you" in a dialogue. This is not necessary in Hungarian.).
Now, here we can leave the "Én" from the translation, because you're telling this sentence to a person directly. Leaving this word is immediately changing "Szeret" to "Szeretlek", which is making it much more personal, plus the "Én" also can be abandonned as it's already appears in this "Szeretlek" as the word itself get it's "I" personal expansion. "Téged" also can be abandonned as the word "Szeretlek" is not just containing the "I = Én", but it's also containing the "you = te", which in this language is rather "téged" due to the structure of the sentence.
So in overall, due to the structure of the sentence, while the 1:1 translation above is correct, as your approach to this sentence is full with emotions, your approach is also correct. So, the translation also can be this, just as you wrote...
I love you = Szeretlek (Also a 1 by 1 translation, but it's the much more personal one).
The first translation above is the 1:1, the formal, the one below is the one full with emotions. But ONLY in that case if want to use it as a seperate sentence. But see the edit part below, why you can't use it on this way, without the "téged".
And my little thief is meant as an endearing nickname for a young woman, who is a thief. Does that change anything?
Nope. In this case the translation is correct for that part.
Other than tolvajomat which must be thief, right?
Actually tolvaj = thief. "Tolvajomat" can be translated as "My thief", but as the words are transforming again to leave all the my, yours, etc, etc... it's already containing "My" in the transformed words. So "tolvajomat" is the correct translation, but it's not meaning "Thief", but it's "My thief" in general. And the little = kis.
But there are various ways to translate this as if you want to make it to a mocking, "My little thief"'s translation is already "Tolvajocskám", which is already containing "My", "Little" and "Thief" in this one word. But this is the mocking, have a quite different, mocking approach in it's tone, so don't use this one.
EDIT: One last thing what I forgot. As you're using "Szeretlek" in an expanded sentence where you're also referring to someone else too, there you have two + one options.
1. "Szeretlek téged és a kis tolvajomat." (In this version you must use the formal as you don't want to give different love to the two different person. You're not making difference between the first person and the little thief. In this case the formal should be applied with the word, "téged".
2. "Szeretlek, és a kis tolvajomat is." (That comma is necessary if you want to leave the word "téged". The comma is giving a weight for the first part, the "szeretlek", and then you can leave "téged". But only in this case if you're intending to use the sentence on the way you planned. At the end, the "is" is meaning "too". So the pure translation for this second option is "I love you... and my little thief too.". If you want the 1:1 translation, you must use the "téged" if you want the sentence to sound really nice and personal for both person. In this second version the first person gets much more love via the sentence and the "little thief" gets a "secondary", less personal, rather formal love.).
Bonus version. The possible expansion is in red, but as we're using some phrase a bit different, the red are not necessary in this version at all (If the reader already knows who the other in the "both".).
"Szeretlek mindkettőtöket;
téged és a kis tolvajomat is."
Dry translation: "I love you both;
you and my little thief."
Essence: no difference between the two person, same love applies for both, and it's also personal. But as I wrote above, the reader must know who to you refer in this sentence. As there are two person mentioned, the "Szeretlek mindkettőtöket" is good.
It happened before I got there, but it had become a running joke by then. Every new person who didn't speak Hungarian (we had a lot of folks from the former Yugoslavia) was told to ask that boss something about cookies