As things stand now, I'm sure that children are already more than welcome to contribute to our existing forums.
However, there are advantages in having a separate space set aside for their contributions where everyone involved would know that the creators were under age. Viewers would know to judge the works accordingly and to tailor their comments appropriately. Forum moderators would be able to give the forum special attention, not only to make sure that commenters behave themselves and that the tone stays particularly positive, but to help protect the children from sharing more than they should about their personal selves. It might even be a good idea for every post in a child-centered forum to have to be reviewed before it is made visible to the public, though I'm not sure if our site is set up for such a thing. Perhaps a special moderator or set of moderators would keep a close eye on it. Likely accounts involving underage humans should have parent/guardian approval/involvement.
All the nitty-gritty practical stuff aside, I think the idea of making out site more welcoming and supportive of younger creators would be a wonderful thing. Children are innately creative, love to share, and benefit greatly from the input and feedback of others (suggestions, praise, and positive criticism), especially from adults. This is true of pre-schoolers through teens. We terrain builders, by definition, enjoy making things just as children do. And we patrons of TG, by our choosing to be here, enjoy sharing our craft and techniques with each other in a positive way. Also, the parents/guardians/teachers/reliatives of creative children do not always have the skills or knowledge to best support their charges in every endeavor. Having a safe space online of knowledgeable adults to help would likely be appreciated (minimal cheap safe tools, project suggestions, clean-up, safety concerns/what products to avoid, resources for materials, etc.).
The terrain building community at large could strongly benefit from reaching out to support the next generation of creators, and younger creators would benefit from the wisdom, support, and enthusiasm of adult builders. Our site in particular seems tailor-made to be at the forefront such an effort.