I ordered one of the line following Robot kits from eBay a while ago and thought I would get it put together this weekend.
The kit can be found at: https://www.ebay.com.au/itm/D2-1-Intelligent-Tracking-Smart-Car-Suite-DIY-Kits-TT-Motor-Electronic-Comp-K2P6/302570466688
It is a pretty easy kit to put together, although I had to have a laugh when I opened the instructions that were in Chinese, not such a good start. There is a schematic in the instructions though so all was well. In fact lucky it was as I needed to do a little fault finding at the end.
The circuit boat=rd is well laid out and there is a silk screen that details where all the components go so it was easy to construct. The mounting of the motors is also pretty easy, the front of the robot is supported by a skid rather than another wheel, which surprised me a little but it was a cheap kit and you get what you pay for.
At the end I found it didnt operate quite rigth and on measuring the voltages on the comparator inputs I found one was not varying with the calibration potentiometer. I found a small solder bridge, bad me!!! Once this was sorted it worked fine.
This kit would be suitable for an older child, depending on soldering skills they may need some assistance and also posssibiliy help identifing individual components.
You can checkout the construction on You Tube at: https://youtu.be/fZ41wADri8E