My dd had to use an inhaler for a short period of time and my GP said to keep the spacer and mask over her face for at least a minute. She's 18 months and hates it too, screams, but the GP said if they scream they take good deep breaths so just to persevere with it. I find it much easier to give it when dp is around to help, when it's just me I feel a bit like I'm torturing her!
Incidentally, she's never really coughed much after it.