Your middle Japanese sentence would be about mid level grammar. I think you've translated it okay, but that かいて isn't in kanji, so it's hard to get the meaning clearly. It's not always best to be literal, but I sort of do that to help with my 'Japanese thinking,' in that if I consider how they say it in English, it helps me imitate them better. I would translate it as "A kid plays blowing a straw and making bubbles has been drawn," if draw is the verb. I'm not all that great with usage of ~てあります。 I get the gist, but I don't use it at all.
The second one is also mid-level. でしょう is not necessarily a question. It depends on the context. I would think it's sort of a rhetorical question in this case, "Why would that be?" or "I wonder why?"
One of the reasons I can't give you a better answer on that ~てあります point is that I stopped studying about five years ago, and the stuff that isn't as common slowly fades away, while でしょう is used all the time, so you sort of pick it up without trying if you speak to natives a lot.
If you are concerned with grammar, try picking up a book like this:
Amazon.co.jp: 日本語総まとめ N3 文法 (「日本語能力試験」対策): 佐々木 仁子, 松本 紀子: 本
That's Japan's Amazon, but I'm sure it's available in the States for not too much money. That's the level 3 JLPT, so it's mid-level, and if you can get a solid handle on that, then you will be much better off. I would say the JLPT 2 is good for a wide variety of Japanese comprehension, and should be your goal, but just take it one step at a time and work your way up