I’m building a treehouse for the kids.  Looking for a solution for a small power source in it to power a few small items.  Also looking for a low cost, simple option.  

Ideally, if this solution exists, I’d like something like a medium size usb power bank, something like a 20k mah usb power bank with 3 usb outlets that I can hook a small solar panel into to maintain charge.  

Ideally it would power a small usb fan, a small 18650 lantern that charges via usb, and have a spare port if they want to plug in an iPad or something to watch a movie in there.  

Does something like this exist as a simple diy or plug and play option?  I already have a 26k mah anker battery pack that would almost fit the bill as far as the battery, but it doesn’t allow power out while plugged in and charging.  

I know that there’s high cost options like a goal zero pack that allows continuous solar charging while devices are plugged in and drawing power, and  I’d like to avoid a 12v marine battery/inverter/charge controller if possible, but I’m open to suggestion.

Thanks in advance!

I bought this setup for my son's "shed" for Christmas.  Does good.  Not sure it it's what you're looking for.  The charger controller only has 2 usb ports though.

solar panel https://www.amazon.com/gp/product/B01M9B6RQI/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

solar charger controller https://www.amazon.com/gp/product/B072MMDY4F/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

battery https://www.amazon.com/gp/product/B003S1RQ2S/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

led light strip https://www.amazon.com/gp/product/B01339G2F6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

The 26k mah external battery should serve you well.  Not being able to charge the external battery and run a device simultaneously can be worked around, especially given the generous capacity of your external battery.   For example, at night you'll run the devices off your external battery. During the day, your solar panel can be charging the battery and with the extra USB in the solar panel run your fan, or help reduce the rate of discharge on your electronic device so it lasts longer on it's own internal battery.

Since you're operating off a power bank, I'm assuming you want to keep the system USB based rather than 12v 'solar generator' based.

https://www.amazon.com/BigBlue-Charger-Digital-Waterproof-Foldable/dp/B071G4CQSR

I wouldn't go any less than that, listed wattage wise.  You'll do good to get 16-17 watts in practice during peak hours (probably less in PA), assuming you can find a place that gets good sun in the tree.  That means about 2 amps out of one USB port and about 1 amp out of the other.

Keep in mind even a little shade kills the output of a lot of the panels, and some don't automatically ramp back up when the cloud or tree branch gets out of the way again, so keep a careful eye on reviews regardless of what you decide to make sure it will.  Otherwise you may come back an hour or two later and discover that because a cloud passed over, your device has barely charged despite it being sunny and clear since.

Im left wondering where the solar panel will go.   Unless youre going to mount it at the top of the tree, or off at a distance out of the shadow, the leaves will block the bulk of the solar energy.  This also assumes the tree will leaf out.

How quickly are you draining a large usb battery pack?

One of the local thru-hikers shelters has a solar set up for hikers to recharge their stuff off of. I wasn't involved in the installation, but I've spoken to the guy who donated the stuff, and he said it was pretty much plug and play.
Nothing real fancy. No fans, etc. Just USB for hikers to plug their stuff into.
Its basically a solar panel mounted on an external post, with the associated controlling box and plugs inside the shelter.
I'm tagging this thread for interest as I think its  a pretty nice concept.

Thanks for the suggestions so far.  If I don’t find an even easier plug and play, this may be the way I go.

As far as mounting the panel, there is a dead, mostly limbless tree nearby that I might be able to mount to.  The treehouse is at the edge of a clearing, so a portion of the area nearby, does get pretty good sun for about half of the day.

Haven’t tried it yet, but the kids kill a smaller 10k easily by the time they fully charge an iPad.  Ideally they would take one down there fully charged and primarily only use the treehouse power for the fan and lights, but practically speaking that likely won’t happen