Calling all tinkerers!
My daughter has a battery powered(3 AA batteries) string light which came with her play tent. A number of times she falls asleep with the light turned on.
I would love to be able to control it by Hubitat so that it can be intelligently turned off. Any thoughts on how I can somehow control it from Hubitat or Alexa?
Does it have a remote or a switch of any kind?
If true, then "maybe", but the interface will require a bit of knowledge of programming and some fairly expensive hardware. If it doesn't come with a remote, then it won't be compatible.
My recommendation is rechargeable batteries.
A remote and an existing Harmony or potentially Broadlink RM Pro may help, purchasing one of these just for this one integration would, as you say, be expensive for the result.
Like @saurabh9 mentions, he is calling on tinkerers. I'd imaging some may be open to some simple wiring tweaks to provide the equivalent power from what expect could be a relatively low powered source.
Similarly, a switch may open up options for SwitchBot, but again, cost would be a bit steep for the result unless there is one already available in the house.
Rechargeable batteries would be good, but in my experience, with even a single lamp, I can only get 5-6 hours I think from 3 rechargeable batteries, so I would think you'd be charging and replacing them each day you wanted to use the lights, which may not be particularly convenient.
Something like this is probably the easiest:
Predictable reply....
Nodemcu. Konnected. Relay. Convert to dc.
Done. That's what I do.
And for what it's worth, switchbot wouldn't get a penny from me. Way overpriced.
Costco had 4 packs of these in-store for ~$9-$10.
They come with a 6 hour auto off function.
Features:
50 color changing LED lights per set
Lighted length per set is 15.7 ft. (4.79 m)
Indoor/Outdoor battery box uses 2 AA batteries (not included) per set
6 hour on/18 hour off timer option
It has a small switch. Does not have any remote. I am thinking of the easiest way to convert it to AC or USB power, so then I can put it on wifi or ZigBee plug.
You didn't say how old your child is but with a play tent which may be subject to being pushed around, there is always the possibility of pulling an AC plug and causing arcing.
Running a string of LEDs from batteries is a safe option. Also battery powered wifi/zigbee (or for that matter, any other wireless technology) switches don't exist for a simple reason.
I would just leave it well alone (or use NiMh rechargeables for power if needed).
For uncontrollable things we sometimes just put a camera facing them so we can check their state (and turn off manually if needed)
A camera in his daughters room might not be an appropriate use of that solution....
You can put a dry contract relay into the line between the battery terminals and the lights. They have to have power (typically USB) to activate the relays, thus, I doubt you are any better off.
Is her play tent indoors or outdoors? If indoors you might consider a Zooz ZEN51. It has a "dry" contact relay. Here "dry" means the contact is isolated from other circuits.
Actually if indoors I would go to home depot and purchase a simple timer switch.
I should mention, you will have to "break" into the string light somewhere to add a switch contact.
I think there are a variety of ways to fix it. Amazon has a bunch of usb controllers listed for different voltages. The problem will likely be you will have to break into the circut with something to control it or completely replace the battery bank controller it has.
I think the concern about how portable it is and how much it is moved is a big one.
OR you might consider a set of string lights with a built in timer like this and replace the current set of lights.
We have one like this. We wait until 5:00PM then turn the switch to timer. After that it will go on at 5:00PM every day for 6 hours. We don't touch it until after Christmas.
I mention this as your lights may already have such a timer.
You'll have to get a small little project box (if I recall also talked about in this thread) to tidy this up and of course the DC power to power it but ..."neither snow nor rain nor heat nor gloom of night keeps this little relay from controlling my DC lights way out beyond the reach of wire" .
You could even set it up where she could use the relay to turn them off/on while HE would automatically made sure they were turned off by time X (just not during the bewitching hour of the annual time changes as I just learned).
Of course, depending on age, those lights might be a nice middle-of-the-night security blanket... and you'd be better off investing in good rechargeable batteries and just leave em be. Until she's a teen, and then the ability to flash those lights, sound alarms, and turn on the sprinklers would all be nice features around that tent. 
You can use one of these and put a zigbee or zwave relay switch to control it. Just place it in-between the battery pole and the case contact....
This is how it works....
Sorry for the delay in my response. Got bit busy at life.
Lots of great ideas here!
To answer few questions,
Pictures below:
I've using a kit like this on 3 different battery powered things (Flume water reader, 3rd Reality zigbee switch, 3rd Reality zigbee contact sensor), and they've been working like a champ.
You could then plug it into a smart wall plug for automated control.
I linked to the 3-AA version here, they make other AA and AAA combos too -- the corresponding power adapter each comes with will determine the right amount of juice to put out (3v, 6v, etc)
Ahh, then a ZigBee wall plug and an old USB 5V adapter should do the trick.
Just have to get from the adapter to the switch in the battery/control box.
Time to break out the soldering iron. 
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