Can the government see what you do on the internet on your phone?

Yes, YOU can get into trouble for HIS misuse.

It is your phone.

YES, the government very actively watches those sites and who goes to them.

Kiddie porn is highly illegal, regardless of the person's age.

Not to worry about the NSA though. They aren't looking for that.

But FBI and even your local cops watch things.

Look up Stingray II.

I'm sure that it was your younger brother.........

Anyway you can't get in trouble for looking that up since teen can be 18/19. You also can only get in trouble for buying, selling, and looking at it. You can look up the words, 'Child Porn' and not get in trouble I belive.

yes they will track you down and arrest you

The government has you on a watch list.  The CIA also has you on a watch list. 

They can yes. But are they actively watching that specific phone? Unlikely. But also some websites are "bait" sites. You go to them, and the IP addresses are being captured and sent to law enforcement. Of course there's no way to know which sites these are. The best thing to do, is do a full wipe and factory reset of that phone. Clear any trace of your activities from it. This way, if anyone from the police does show up. They wont be able to prove it was your phone. At least not in any way that's easy enough for it to be worth their time.

your younger brother shouldnt use your phone anymore.  why your parents doesnt know this. you had told them that your brother had been looking at porns sites its danger for teenagers. because he can mistakes to push some thing like a virus from the bad sites onto your phone. and you get a lot of inks from unknown sites its hard to get rid of them on the phone . next time you should just hide your phone or turn if off or upset a code on your phone that you only you can open the phone and your brother wont able cant get on the internet or get access when he does touch your phone again. he may have to finger out what the code is. i have a code on my phone because i have 5 -6 year old nieces who love to play iphone so i set up my own code so they cant able open my phone . No the goverment cant see or able review your phones at all only if your in trouble by a law or in danger and cant find out to get help the police departments and fire frighters stations can able track you down in case some thing happen to you like you were lost and cant get back to home, or was kidnapped and or ran away from home or did some thing illegal shouldnt be doing it yes the police departments can track you down. but government nope. why you asking that. you cant get in trouble for having younger child looking at someone else phone. but every porn sites had a stricity age 18+ older to able to enter the sites but you know that porn sites some are fakes not really real girls having sex. but there are some are real people that making money of it but it better keep your brother off it because if he is just ages 11-under 15  at least yes he can get into trouble not you. him because if he enter those sites online phone or laptops there some are can able steal informations about you and your family members like bank account, passwords, personality informations about who you are. then if they steal  your informations they will make threat calls and send you bad porns sites that you dont want to deal with and there are so many of them that theyre alot of gays, bi ,lesbians and can sell your informations to other porns sites  to sell your data on phone and laptops. on phone company carriers you always paid for monthly they can collect your data without asking permission from you. you need to be care fully with those people on bad sites. you need to keep your phone away from your brother .if gets worst then better to tell your parents let them take care of your brother. 

Yes. Your own Government and law enforcement agencies. As well as the US, Russian and Chinese Governments.

Despite what some think, the internet is not secure or private.

However, I doubt the FBI will come knocking over that one search. Just be careful in future and if you must search for porn avoid keywords like "teen" they are the ones that raise red flags. Use keywords like "mature women" or something that show you are looking for adult legal porn. You also need to be 18+ to look at porn btw.

The FBI is tracking all under age porn surfers and yes they can easily trace it back to your phone and they will arrest the one who is paying for the phone

I don't think the porn police are out monitoring people's phone. Same for the sex police.

Yes they can. It can also be hacked. Just reset the phone.