When our children were that sort of age we used to sit them in their car seats. They were quite used to this and unable to undo the belts. There should be no problem in attaching the car seat to the aircraft using the lap strap. As to keeping them amused just use the same supply of books/toys they would have in the car.
At the time I had a share in a Robin 100-210 and with two children I would have one in the co-pilots seat alongside me and Mrs approach would sit in the back with no.2 child. However, you should put the co's seat as far back as it will go as one of the Missapproaches at 18 months found it very amusing to start kicking at/pushing on this interesting moving yoke thing in front of her. our children were quite happy to fly on legs of up to around 2.5 hours.