Firstly, thank you for your support in bringing rainbow identities into the classroom! It will make a huge difference to rainbow kids in your class (or those with rainbow parents) to see themselves in the world, and know they are OK, and can thrive, shine and grow freely.
Some young kids already know they are transgender, or nonbinary, or just 'different somehow'. Others will grow into it later. Either way, little conversations let them know they are OK.
In general, we find kids are totally open to these conversations, as long as you use age-appropriate, factual language. A child that says 'but boys don't wear skirts!!' isn't being prejudiced, they are just trying to understand the world. They will probably be satisfied with 'some boys don't wear skirts, but some do'. Likewise 'but all boys have penises' can be met with 'most boys have penises, but not all'.
Whatever comes up, keep it simple and factual and you'll be fine.
Sometimes people think that we at InsideOUT are trying to label kids and make them grow up too fast. You might face the same objection. However, what we want is for kids to be able to play, try out wearing a skirt or sparkles or a silly santa beard if that calls to them, try out 'he' or 'she' if they want, and in time grow healthily into whichever adult they are.
Five year old Sam might grow into being Samuel or Samantha, and might fall in love with a woman or a man or neither. Either way, we want Sam to thrive, be free of bullying or shame, and able to make their own choices.
Thank you for gently introducing all kids to the fact that rainbow people exist, and are all good.
InsideOUT for being so cool and running workshops to explore this kōrero more.