Hi,
 
I think that SEWP was created like it is, partly by fear of creating a project openly directed to children, and I'm afraid it precisely make it not so compelling for them.
How is Simple English Wikipedia not compelling for kids? Define “not compelling.”

Well, I didn't closely study SEWP, so I may be unfair, that's rather my impressions :
The main rule is the simple language rather than explain a subject in a accessible way, so you can have long and not so accessible articles in simple language.
When I had a look to the most viewed articles (not exactly recently) I had the impression that it actually reach one of its goal that is to be a resource for non native english speaker (that moreover don't have a well developped Wikipedia in their native language, such as indians of a minority language). Yet it wasn't children interest or school curriculum subjects that were much viewed but rather adult topics.
 
I took a look at Vikidia, thought I could do something for them, signed up with an account, read what I could find of the guidance, and created an article on Underwater diving, following the rules as I understood them, using properly attributed CC-by-sa content from Simple English Wikipedia as a basic framework, and was busy expanding it when it was deleted without discussion by user Ajeje_Brazorf with the edit summary (Please don't copy from simple wikipedia}, and no explanation why not. If I had done that on English Wikipedia I would risk losing my admin bit. If that is how new users are routinely treated there that encyclopedia is doomed. I will not be back to waste my time there.

By the way, sorry for that, I guess it is not specific to Vikidia in english to experiment such diagreement with newcomers, there is a discussion on this wiki about what happened.
 
On top of that, it would seem that you were able to sign up easily without verifying your age; both verifying age and not can be problematic. 
 
Verifying age makes things safer in terms of everyone knowing that if someone says they’re a kid, they’re a kid, and if they’re an adult, they’re an adult. Part of online grooming and police sting operations is people saying “I’m a kid, let’s meet offline." It can be problematic due to reasons of privacy. It’s not like you want everyone knowing the age of a child who’s online. That can mostly but entirely alleviated by only having the administration know everyones’ birthdate, and forcing admins to have strict account security.

We don't ask nor verify the editors ages on Vikidia. There is no status attached to the user's age (with some few exception like check-user...), so few reason to lie about our age if we tell it. However, regular users have in mind that the age told by any user isn't verified. We disable the e-mail function from one's user account to another, so there is no private message unless one editor would have displayed his e-mail adress or on account on another social media. And we suppress identifying information that a child would write on his user page.
To be complete, we noticed that the UK Safeguarding Vulnerable Groups Act 2006 appears to mandate some verifications for some status like admins on a wiki for children (someone that would become administrator of Vikidia in english if he is more than 16 and lives in the UK would have to provide his identity and proof that he has passed the british vetting process). See https://en.vikidia.org/wiki/Vikidia:Legal_matters#UK_Safeguarding_Vulnerable_Groups_Act_2006
 
Actually, Vikidia in english does exist, with 4,035 articles !
 
Oh, okay, so it’s going to be mostly out of whole cloth, not entirely. I’m not sure how many articles English Wikipedia has, but I know it’s in the millions.
 
They may be a way to promote a existing (or to be launched) wiki encyclopedia for children, to "use the momentum of Wikipedia to make it easier to discover”
I believe I’ve stated more or less stated that you can’t use our momentum since we need for the projects we have.

That's a constant pattern, your favorite wiki may have 500 or 5 millions articles, 2 or 20,000 active users, you allway feel you lack content and editors ! ;-)

I mean that if by chance someone or an organisation would want to developp a free documentary resource for children and would be eager to invest in it, there would be a way to do it: there is allready a base of articles in some Vikidia versions like the english one and an organisation that proved to be sustainable (actually very closed to the one of Wikipedia), then some funds would allow to catch up the time of "organic growth" that some wiki encyclopedia for children had. I guess it would be promotion, communication, and possibly paid translation to get the most interesting, best quality, most usefull articles from several wikis for children and from Simple English Wikipedia (with some data analysis/study to identify and select them), to get a core of say 2,000 or 5,000 or 10,000 articles in one language. It would be quite a big operation, but it may be worth the investment. Getting a quality free encyclopedia for children in a matter of months.