So, a bit of armchair psychology.
At this stage of a million new skills being learnt every week, little toddlers almost universally want to please their mums. That's why I mention doing lots of independent movement and instruction following during the day - to develop that pattern of:
Instruction > you pause with expectant (happy) face > instruction followed > total independence of movement > big smiles, cheers, praise and cuddles from mummy.
Children are not born with negative behavioural issues like defiance - these are learnt. And at 18 months, it's too early to have learnt it. They are learnt by repetition if finding that defiant behaviour gets the result they want. You are aiming yo teach the opposite, the positive version. Following instructions means Mummy is very pleased with you, toddler loves that praise so the feedback loop begins.
I would use facial expression a lot. For example:
- normal pre-sleep routine. Say milk downstairs, upstairs for night clothes, cuddle and story in bedroom...
- Put into cot stood up (every single time, never ever lie him down)
- tap matters, say lie down
- Swap to "No! It's sleep time now, we lie down to sleep. Lie. Down." Tap matress
- . Give him time to decide to follow the instruction.
- When he does, as soon as he starts making a movement towards it, immediately change to . Lots of positive reinforcement, praise.
- Hand on chest to settle him once he does lie down. Keep the positive, compassionate, expression with eye contact.
If he was to continue to not lie down, I'd roll back the routine. So out if the cot, straight onto the floor. Bend down to his eye level - sturn face, repeat your "No! It's sleep time now, we lie down to sleep. You must lie down". Change to a positive facial expression and say ok, well try again.
Start again. Into cot stood up, tap mattress, tell to lie down, positive expectant facial expression, wait.
If that's not happening, roll back further. Maybe go back to cuddling with a story in his room in order to calm him down and reset the positivity. Read until he's calm and start again. But do the same - into cot stood up, tap matters, tell him to lie down.
It's a case of having very firm boundaries on this. You will expect him to lie down at bedtime. You are not forcing him to sleep but you are expecting him to lie down and stay lying down.
It's the tough bit of parenting. Essentially the same sorts of things you do with the Terrible Twos and Threenager tantrums - you set certain boundaries that are hard boundaries and not negotiable. DC may try to push against them but you stay firm. Pretty quickly DC accepts that your way is the only option on this one.