Do you have characters — or stickfigures
Years ago I attended a presentation by Emmy-winning author, Sean Stewart, in which he explained why there were no children in SF. It is, he said, extremely tricky to keep the action going and the tension up if the heroine running down the corridor is trailed by a toddler saying "Are we there yet, Mommy?" or "I don't like the Death Star, Mommy! I want to go home!" every ten feet. (I thought this terribly funny at the time, but found it less so when I subsequently had children of my own, and recalled Sean's description as my five-year-old kept punching me every 30 seconds because we were stuck in an hour-long line up at customs one 3AM flight, and she couldn't understand why I wouldn't let her leave.) Kids and action adventure stories do not make an easy mix.
Challenged by Sean's talk, I chose to include a 9 year-old in my own first novel—which may partly explain why my first novel took so long to finish. Although a key factor in a couple of scenes, figuring out what to do with the kid for the rest of the book was ridiculously difficult. Arranging for various babysitters to show up so my hero(s) could go adventuring without him wore thin pretty fast, and the biggest flaw my editor identified in my preliminary draft was that I had simply forgotten about the kid for five chapters while the main characters dealt with their current crisis. "And where is her son when all this is happening?" came to be the one editorial comment I dreaded most during revisions. So yeah, I don't recommend including child characters in an action novel unless one is a glutton for punishment.
The biggest problem I see with child characters coming across my desk as an editor, is authors getting the ages wrong. As any parent knows, there are huge gaps in sophistication between an infant, a toddler, a grade 1, a grade 4, and a grade 7. When one has an infant of one's own, one can accurately peg the age of other infants to within a few weeks. By the time our child is in grade school, our accuracy is down to being able to say if a newly encountered child is the grade above or below our own: a mere six weeks makes no difference developmentally, but a year's difference is still sufficiently significant to be obvious. As kids grow up, age becomes increasingly unimportant, with some teenagers, for example, presenting with greater maturity than many of the adults one encounters. As our own children age, our ability to remember what characteristics go with which age becomes less precise, because that knowledge is no longer relevant to our daily lives—at least not until our children start delivering grandchildren.
Writers, however, need to get this right. If one gives a nine year-old character the dialog of a five-year-old, one's adult readers might not notice—it's just a kid talking—but a nine-year-old reader will find it infuriating. It is not just not credible to that young reader; it is highly insulting to discover the author has so little regard for nine year-olds—whose self-image is that of a grown up / sophisticated almost-adult, definitely not to be confused with a five year-old child. That the writer could make such a fundamental mistake is to them an insurmountable barrier to finishing the book, no matter how good otherwise. Could you finish a book that gave the character of a Sudanese immigrant an Irish brogue? If the author can't get the dialect right, how is the reader to take anything in the novel as credible?
Most authors know better than to attempt depicting an accent they are not themselves intimately familiar with, but I am astonished at how frequently they will assign dialog or actions inconsistent with a character's age when depicting children. If one is writing a YA novel, for example, the younger siblings in the novel had better behave in a credible way, or the YA readers will throw the book across the room in disgust. YA readers have a much more accurate estimation of maturation levels than most adults because they either have actual siblings that age, or have best friends with siblings that age; and if they cannot picture their little brother or sister saying or doing that, the story loses all credibility. I don't understand authors who spend months researching police procedure or forensic evidence or the astronomical details of their SF setting, but are three years off the mark in depicting the reactions of a ten year-old. If one doesn't currently have a ten year-old in one's household, or a convenient nice or nephew, then why even have that character in the novel? If there is some compelling reason to add a child, do your research: go find some kids that age to talk to. One needs to put at least as much research into that character as one would any other element of the novel.
I started with a seven year-old in my novel...but have revised the age upward with each subsequent draft as my own youngest matured, because the only sure test I have ever had for the credibility of that character was to ask, "Is this something my kid might actually say/do in these circumstances?" Of course, not living on a starship, I have to do a certain amount of extrapolation, but at least I'm determined to be in the right ballpark in terms of maturation. Am shocked and appalled how often this is not the case with many of the manuscripts that cross my desk.