The Separation-Individuation stage of early child development is described in an earlier post here. To be very frank, Petunia is no expert in early childhood psychology. I know enough to keep myself out of trouble with my kids, but not really enough to call myself a true expert. I can, however, share what I do know... and what worked for me.
So, for what it's worth, here goes.
Throughout the Terrible Twos, Traumatic Threes, Fearsome Fours and Feisty Fives, I was the famous Chinese military strategist, ZhuGeLiang, personified. I lied and cheated my way through those periods of my children's development. Kids this age are crazy I tell you. They cannot be reasoned with. Firstly, they're too young to possess much reason. Secondly, they can't even talk properly. Thirdly, in the grip of separation-individuation, they will fight you to the bitter end. Unless you are prepared to roll on the floor and create a scene in the way these kids are enthousiastic about, don't get into a fight with them.
If you do meet tantrums, you have to face down EVERY SINGLE one. This is because ONE single capitulation to a tantrum tells your kid that tantrums work on you, and that will then encourage your child to throw even more tantrums. As more tantrums are thrown, some tantrums will work and others won't. The kid can't tell WHEN tantrums work and WHEN they don't... just like a jackpot addict can't tell when he'll hit the jackpot. As a result, the child will be very persistent in throwing tantrums JUST IN CASE, just like the jackpot addict is compelled to pull the lever of the jackpot machine.
Once in a while, I had no way to avert a tantrum. I made sure I faced down EVERY single tantrum. See here for a memorable Little Boy tantrum. I never allowed any tantrum to win a coveted prize. Then I explained gently that if they asked for things so rudely, it was a 100% guarantee that I would NOT give them what they asked for, even if I had initially wanted to. If they wished for anything from me, they had to ask and reason like humans, not little animals. As a result, Little Boy once came to ask reasonably for permission to stay up very late after having thought through his case. He made very cogent arguments and I was impressed. He was allowed to stay up late merely on the strength of the effort he made to reason things through in his funny squeaky voice.
I was on a mission to humanise my son.
There were many avenues for subterfuge. The most useful one was The Meow Took It. Then there was some advanced sweeping of the premises for possible items that might lead us into conflict. Potentially controversial items were removed. I tried not to say "No". I said "Maybe" or "Later" or "We'll see." In the case of apple juice, which Little Boy adored and could drink up to 2 litres a day, Grandma advised me to nod and smile, and then cut his apple juice with progressively more water. Eventually, Little Boy was happily drinking 2 litres of water spiked with 2 tablespoons of apple juice, a day.
Luckily for us, kids at that age are gullible. What they lack in reasoning powers, they make up for in gullibility. If you tell them The Meow Took It, they'll believe you!. You can't reason with a psyche intent on separation-individuation but you can trick that psyche into complacence.
Then, Grandma said to me something profound. The human will hardens when it goes through fire. If you have a strong-willed child and you repeatedly engage in parent-child showdowns, you will harden that child's will and set it into hardness for the rest of his life. This is a recipe for long-term parental suffering. It also does your child a disservice because his strong will becomes hard and brittle (like a steel rod)... when it would actually serve the child better in adulthood if it were strong and flexible (like steel cables).
Grandma believed that it was far more important to harness and channel the strong will, rather than harden or break it. Grandma had no PhD, but I am forever grateful for her wise words, and I had faith in her words because Little Boy takes after The Husband. Grandma had done a great job with tempering The Husband's strong will and rendering it flexible without breaking its strength.
Once they get to an age where you can reason with them, then it's time to stay scrupulously away from subterfuge in order to preserve trust. The development of logic and reason also comes with it the ability to think critically. You don't want your child to think of you critically as a crook.
Before they get to the stage of separation-individuation, they ummm... have only one language. Crying. They have no idea how to ask for things nicely so it's ok to give in to their crying. If we ignored our babies when they cry, many might starve to death.