I also found a readymade solution on Etsy. Seems like you’re not the only one who wants to join two or more LACK tables together.
The LACK Lock slides over two legs and is held in place with an integrated, low visibility screw. They can even make the locking adapter in a wide variety of colors to match the colour of your LACK tables. The lock can be removed when your son has finished playing. But maybe not as easy to disengage as Velcro or barrel bolts.
Let us know the quick and easy solution you end up with.
Hacking may compromise the structural integrity of the item, so please be aware of the risks involved before modifying or altering any IKEA product. Alterations and modifications will also void any warranties or return policies you may have received from IKEA. IKEAhackers.net is not liable for any product failure, injury or damage resulting from the application of suggestions, ideas and hacks featured on this site.