Yep, tried all of that (said child is now 14, so no longer applies!). When you have 4 children under 6 you can't always leave earlier & be more organised...
We would get in the car, strap everyone in, set off up the road & hear "click". Stop car, do up straps "straps stay done up", set off, click. Didn't want to sit in car seat, wanted to climb all over car.... no amount of distraction, food, promises of rewards for good behaviour ever worked. What did work was a smack, combined with "I told you straps stay done up. Leave it".
Other 3 understood that car= straps & never attempted to undo them.
Better a smack than going through the windscreen.