Why do they want immigrants to learn English then make our kids learn a foreign language in school?

It's simply called education and knowledge. And both are very useful.
In my country (Argentina), everyone studies a foreign language since 1st grade at school. That goes on even throughout university.
If you wish to get a Master's degree or a doctorate, you will be required to speak at least two foreign languages; one romance language and one of a different origin. That's in serious universities.
Even if you expect to get a good job you will be required to speak at least one foreign language (not necessarily English).

I read some research years ago that showed Americans as the people with the least general knowledge of the world, the the most monolingual individual.
I wonder, why not learn as much as you can? Linguistics is a science, a well-established one. Language and languages are important and necessary. Even more, they are interesting and will tell you a lot about the people who speak them.

What you said about age, learning a foreign language is lots easier when you're a child. There's a something called in Spanish a "window period" (I am almost certain this is not the exact English equivalent), during which a child will acquire, more than learn, a foreign language effortlessly. This "window" starts to close around the age of 8 or 9. So, the younger children start learning a language, the better.

"Reading, writing and doing sums" is far from enough these days. You speak of social studies, and languages are part of them. People need to become aware that having been born in a developed country alone won't give them any advantages over other people who, perhaps coming from the third world, have more knowledge than the others ever will. I personally find it sad to get answers such as "it's a country in Peru", or "Buenos Aires is the capital city of Brazil" from many foreigners (amazingly, not from Europeans, though), when I ask them if they know where my country is.

Let your children learn as much as they can. Let them know there is a world beyond their own house, and that it's a world full of wonders ready to be discovered and re-discovered, again and again, every time a pair of eyes looks at them for the first time. I don't believe in "useless information". Knowledge is a wonderful and useful tool, if you know what to do with the information stored in your brain.
And there is always something interesting and valuable to do with it.

C.

People have to learn the local tongue when they immigrate to a new country in order to be an asset to the workforce and to help assimilate themselves into their new country. kids have to learn foreign languages because they need to know more than one language to expand their career opportunities and to make them more open to other cultures.

Language is a big part of culture.

I can see both sides to this. People forget that, for immigrants from non-English speaking countries, English *is* a foreign language and culture!

On the other hand, learning a language methodically from scratch develops analytical and cognitive skills, helps memorization, etc. Also, the more languages you know, the more of an asset you'll be in the business world.

Immigrants learn English to become a part of our community. We learn other languages to become a part of the world.

I agree with William. Language is a big part of culture and knowing a second language can give you almost endless opportunities. A lot of jobs will actually hire you if you are bilingual alone.