There's a huge amount of material that falls under the umbrella term "guerilla warfare."
I recommend you start by reading the Wiki page on the topic and follow some of the links.
As to effectiveness, it can be a very effective strategy, especially when it is a long term strategy. In that case the goal is to wear away the enemy (the stronger force) over time so that eventually a political victory can be achieved. This political victory might be the enemy deciding that the losses are no longer worth the effort and they leave the battle (the U.S. in Vietnam) or that civilian pressure from the public or the international community finally forces the enemy to go to the bargaining table to negotiate a settlement (the "Troubles" in Northern Ireland, the fall of the Rhodesian government).
By "long term" think years or even decades, not weeks or months.
To be truly sucessfull the guerillas need the support of the population and the view towards the long term. Outside support for other stronger parties (other countries, an enemy arm smuggling in arms) is also very important.
In the modern age a small group of people can bring a whole city too it's knees. Look at the results of the Boston Marathon bombing where the whole area was on lockdown looking for just two people. Look also at how the LAPD over reacted when former officer Chris Doerner threatened to kill police and their families.
Those were both unconnected incidents. Now imagine if each of those was done as part of a loosely coordinated series of attacks executed in a short period of time over a wide geographic range. You could really stretch enemy resources thin.
Communication among resistance fighters is important as is operational security. Do some reading on the "cell structure" for guerilla/terrorist activity and the concept of "leaderless resistance." The idea is to limit the damage to the movement whenever someone involved is captured. You can't tell what you don't know. This is why the NSA is tapping everything, but there are still ways to communicate.
As far as specific tactics, guerillas always want to apply violence in the time and place of their chosing. Running out to the woods and playing "WWII resistance fighters/Red Dawn" is a good way to get killed by troops with superior numbers, technology and firepower.
Instead a more realistic approach would be bombings targeting symbols of the government, government officials, and vitally important infrastructure targets. Targeted assassinations of enemy leaders or enemy soldiers, law enforcement officers, or even collaborators is also a valid tactic. (From a military POV, not a moral POV). Look at the IRA in the 70's.
Now if your scenario is just a group of 10 to 20 teens, with a few personal weapons, going up against an established government with an effective military, in small scale fights, yeah, they are going to die, and quickly.
Your heros can be part of a larger movement, and still be the stars of the story, but if they ARE the movement, they won't have enough support or live long enough to be effective.
EDIT: As to the enemy being willing to kill kids in the city, the effective guerilla emerges from the population and blends right back into the population as needed.
If the enemy doesn't know who is a guerilla, and who isn't, they can't just kill everyone in the city hoping they get the "right ones." Look at how the VC operated. They'd often have jobs working with American's during the day and would don the black PJ's at night and lob mortar rounds at U.S. camps. (That's the archtypical story, but there is truth there).