What you wrote is okay, but the original emphasizes that he’s sort of settling. He’d rather be a prince, but can’t, so he figures seeing one is the next best thing. 就 is a hard word to explain in simple terms as it has so many usages and no real direct English equilalent, but you could likely translate it here as “just”. As for the 已经, we do something pretty similar in English. It highlights the fact that seeing a prince is something he already can do. Compare: Seeing a prince is pretty good Just seeing a prince is already pretty good. Both are fine, but the second one reads like you’re trying to comfort yourself rather than simply making a statement.

1 在这句话中，小朋友知道他不能做真正的王子，这是一个限制性的条件(It's impossible for him to be a real prince)。但是，如果能见到真正的王子，就已经很不错了，也就是说见王子一面会让他觉得满足！ 2 "就"我认为在这里是表示在某种条件或情况下自然会怎么样！在"就"的前面通常会加上"要是"，"只要"或者"既然"之类的词！ 例如，您给出的这句话可以写成1"只要能见到真正的王子一面，就已经很不错了" 2或者"要是能见到真正的王子一面，他就已经觉得很不错了"！ 在这里，见到真正的王子是一个假设的条件，让他觉得满足是在这个条件下自然产生的结果！ 这是我的看法！如有任何意见，欢迎提出！

你可以说“能见一见真王子是很不错的”或者“能见一见真王子已经是很不错了”，但你注意，这么说话是直截了当的陈述，语气不强烈，没有让步的意思。 “能见一见真王子就已经很不错了”有强烈的让步意思，意思是说，现在小朋根本没见过王子，先要解决的问题是见一见王子，，如果他想做王子，那是异想天开，贪心太大，癞蛤蟆想吃天鹅肉！

As a Chinese it's pretty hard to explain the general meaning of 就 to you. Sometimes it's preposition, adverb, conjunction or verb. As for this sentence, 就 means "then" (adv.). The whole sentence "就已经很不错了" you can say in English "then it's already good enough" .

你好Morris, I'm a Chinese learner as well. I usually use the expression when something expected gives a rather good result. 就已经很不错了 means it's good enough. 就 here is used to emphasize something being mentioned. It implies a convincing nuance. 已经 + 不 + phrase + 了 is used to express something which is expected has reach to a certain extent. i.e.: 天气已经不热了，咱们去散步吧。 As for your question whether you could say 是很不错了，it's acceptable. But, the tone isn't as convincing as 就很不错了. My Chinese professor once told me that Chinese languange is about 语感， which means the feeling of language. Surely, there're a few grammar rules and collocations that we can approach by using English learning method. However, there're some that you have to feel it and get used to. That's my understanding. We'll see how the natives view it.