If you have a toddler you know that those little people go from a squishy newborn to a roly poly baby to a nay saying toddler in the space of hiccup. That once-portable non-opinionated being morphs into a walking, talking contradiction.
Basically, the more you want them to do something the more they will fight against you. Need to put the toddler in the car seat? Stiff board is a nearly unconquerable obstacle no matter what you try once the battle escalates to this point. Want your child to ride in the grocery cart? You will start to wonder how many legs they actually have.
The most successful strategy for getting cooperation from my kids as toddlers has two parts.
First, tell them the expectation before you get to the car or the cart. "D is going to ride in the cart in the store today!" It's important that you phrase this as a statement and with an excited and happy tone of voice.
The second step requires you to make a fool of yourself. Sing a song, make up a chant, make silly faces, whatever...while you are putting the child in the car seat or cart or whatever. Distraction from the act of "making" the child do what you want him/her to do is key.
It doesn't work always. Sometimes I'm too rushed or forgetful to remember to prepare my toddler beforehand. Sometimes I just don't want to sing.
But if I do the steps and sing the goofy song? Works like a charm.