I concur that the dobsonian mount would be your best bet. This mount is usually very simple and you can almost certainly design one customized for your scope from scratch after looking at a few examples on the web. This mount is what is referred to as Alt-Az or altitude-azimuth mount. The up-down bearing can be simply two wooden disks mounted 180 degrees apart on opposite sides of your present tube at the center of gravity point where the tube wants to balance with an eyepiece in place. These edges of these two disks ride in half round cutouts in the sideboards of the lower cradle box. You can add a second piece of wood with a half round cutout above the disk baring to hold the scope captive in the mount but most people don`t, relying on gravity to do the job. This makes it easy to lift the tube assembly out of the mount for separate handling during transport and storage. These cutouts are often lined with Teflon or some other slippery plastic to allow smother motion. To provide for movement on the side to side (azimuth) axis, you can use one of those commercial "lazy suzan" ball bearing assembles available from home center hardware stores. This bearing is mounted between the bottom of the lower cradle box and a plywood plank base which sits on the ground and allows the whole cradle box to spin around 360 degrees. That’s pretty much it. You don`t need super precise plans, and in fact could get into a little trouble by following some one else`s exact dimensions when mounting the disk bearings to the tube since the mounting point is selected to insure that the tube stays reasonably well balanced on the altitude axis. You may also want to scale the whole thing to place the eyepiece at a convenient height for your kid’s viewing. For example you could move the disk bearings down towards the mirror end of the tube and ad weight near the mirror to restore balance. This would lower the hight needed for the lower cradle box because less clearance would be needed for the lower part of the tube to swing clear. This would inturn lower the eyepiece hight slightly for lower viewing angles because the pivot point would be lower (wouldn`t help much when the scope was pointed straight up near zenith). Just do a web search for "dobsonian", look at some examples, and take a crack at it.