"I saw your child."
The accepted answer here is inconsistent with the accepted answer to 'Have you seen her book?', which is '你看见她的书吗？ '. When I tried a similar structure here, without 了, it was marked wrong.
I am finding the lack of consistency between questions, particularly in this module, very frustrating and confusing. If the DL accepted answers in both cases are in fact correct it would be most helpful to have some explanation. On the other hand, if my answers are also correct, it would be nice to know instead of being marked down.
The current situation is in my opinion a negative learning environment that just sows confusion and uncertainty.
I can understand your implication, and I do think Duolingo should have a tips section concerning "了 (le)".
In any case, to answer your concern with using 了, it is not a term to indicate past tense, it's a marker to say an action is completed. In "我看见了你的孩子", the act of seeing your child is completed. In the sentence "你看见我的手机吗?" the question is not necessarily saw my cell phone already and completed the action, "你看见了我的手机吗?" would be like if I asked if you already saw my cell phone. Different implication and does not really help me currently looking for it. I know it's a confusing matter that doesn't really translate well between these two languages but I hope I was able to help.
看 is closest in meaning to the verb "to look" whereas 看见 is closer to "to see", i.e., the latter includes the implication that something was actually seen. For reference:
The "le" can be where it is or at the end. This sentence doesn't have enough context to have only one correct placement for "le." If, for example, I had planned to see the person's child for a meeting after school, then I would put "le" at the end of the sentence, to signal the planned meeting was over.
I don't have a solid answer to explain all instances of using or not using 了, but I think I can give you a general idea. Bottom line: 了 is not a past tense marker. 了 can be a completed action marker. Putting 了 immediately after the verb emphasises the completed nature of the verb.
I believe English sentences with "see," "saw," and "did you see...?" will use 了. These sentences describe a particular instance of the action. We are describing seeing that happened in a particular, definite instance. Here 了 will go directly after the verb to show that the action occurred and is completed.
English sentences with "have seen," or "have you seen...?" will not necessarily have 了. These sentences might refer to something being seen over a long period of time, or many times, or the emphasis of the sentence may be on the object that was seen, not on the action of seeing. Here the "completed action" 了 may not be appropriate (and so the sentence might not have a 了).
The word '了' is like 'already' in English. It is often used to stress that the action is complete. Nothing concerns time or tense actually. It is often omitted when such completeness is not the crucial information. In this case the translation should be "I saw your child already." to guide that there is a stress of the aspect. "I saw your child." alone reflects the aspect but there is no stress, and that is quite different.