Amy is starting to learn to use the auxiliary verb do in English—for example, in questions ("What do you see?" instead of "What see you?") and in negations ("I don't see a fish" instead of "I see not a fish").
She's not using them much yet, but what she has started to do is use "do" as an auxiliary for the past tense!
For example, when I came home, she pointed into the kitchen and said, "Look what we did buy". Or she might say "I did saw that".
I can imagine that it's influenced by not having to put the verb into the simple past (though she'll sometimes do that, too, e.g. "did saw") but being able to use the infinitve; and possibly also by the fact that the most common past tense in spoken German uses an auxiliary.