Considering the state of Cybertron and the Transformers race in the live-action series, being a Child Soldier is one of the least of their problems.
Keep in mind that pretty much all Cybertronians are millions of years old. Skids and Mudflap are likely just immature.
And the novelization of the third film has them killed by Sentinel Prime.
Soundwave's been on Earth since the nineteen-seventies, at least, keeping an eye on humans, and killing with impunity. And we never hear what happened to John Keller, Leo Spitz, Maggie and Glenn, did we?
Compared to much of the other Transformers "universes", there is little talk in the films about energon, aside from a few references to "Energon Detectors" and as a replacement word for "blood". (i.e. no allusions to secret stashing of energon off Cybertron, the existence of natural sources of energon on Earth or even the manufacturing of synthetic Energon that dominated G1.) Starscream even mentions in Revenge of the Fallen that the Energon shortage is making reproduction impossible. Given that Cybertron is a dead world, and the only known remaining source of Energon is the Matrix of Leadership '' does this mean that aside from the 9 Autobots on we see in the film, the entire Autobot race is slowly starving to death?
Some fans might have been put off by how Optimus Prime killed Megatron in Dark of the Moon, because that is not how fans see Optimus. When you look over at Transformers: Prime, and see how Optimus was intent on killing Megatron after Raphael is nearly killed, the scene's brilliance comes into play. Because of Megatron's actions over the course of Revenge of the Fallen and Dark of the Moon, Megatron had gone too far in Optimus' eyes, just as his Transformers: Prime counterpart did.
On another side, considering that Optimus's master, Sentinel, teamed up with Megatron because he thought that the Autobots were never going to win, Optimus probably refused because then he would be no better than his master.
The Fallen was planning to betray Megatron. If the Solar Harvester went off, it either would've blown up the sun, which would most likely wipe out the Earth, or the entire planet would freeze really quickly. No wonder the Decepticons, who had several decades to search for it, never seemed to find it.
In TF1 Megatron went after the Allspark by himself leading him crash at the Arctic. In DOTM Megatron states he and Sentinel Prime was supposed to rendezvous on Earth but fate waylaid them both. You realize that Sentinel Prime gave the location of the Allspark to Megatron. It was probably Sentinel Prime's Space Bridge that propelled the Allspark to Earth in the first place. The original plan was for Megatron to retrieve the Allspark while Sentinel Prime brings over Cybertron via the pillars. Together they would end the war and rebuild Cybertron.
The Fallen thought Optimus Prime is the last Prime as he felt his death. Optimus was not the last Prime, there is another, Sentinel Prime. The Fallen could not detect Sentinel Prime as he was in stasis and thus assumed he died while trying to escape Cybertron in the Ark.
Megatron serves the Fallen yet it turns out Megatron has an alliance with Sentinel Prime. The Fallen wants every Prime dead as they can kill them. Thing is Megatron never intended to be second fiddle to the Fallen. In ROTF Megatron stated "Even in death, there is no command but mine." and in DOTM "All I want is to be back in charge". Megatron wanted to use Sentinel Prime to get the Autobots to surrender and use him to kill the Fallen to secure undisputed leadership of the Decepticons.
The Transformers are aliens capable of downloading and speaking Earth's languages. But remember they still have their own native tongue. Even among Earth languages there are words or concepts that do not translate very well from say, Japanese to English. If we can get these kinds of translation difficulties arising on the same planet, then it's almost guaranteed that the culture and language of an alien race is going to have words and concepts that are very hard, if not impossible to convey properly in the span of a few seconds. So when the ancient Primes told Sam that Optimus was their last "descendent" they probably didn't mean he comes from their lineage in whatever passes as Cybertronion biology. They probably just chose that word because, based on the 3rd definition here, it was the closest they could get to a literal translation.