Involving your children in your ceremony is a sweet way of making them a real part of your wedding day. There are many ways in which children can be involved - my biggest piece of advice though is to expect the unexpected and just go with the flow. Children don't always behave as expected, especially in a new setting, but that's okay. Just adds to the charm of the day.
Some ways you could involve your children:
The traditional flower girl/page boy roles may be oldies but are still goodies. Children could carry a sign saying, "Here comes Mummy" or similar.
Ring bearers. The rings could be tied to a favourite toy as well as the traditional cushion.
Have children do a reading - a short, lighthearted poem would be perfect. Your celebrant would have some ideas on what may be appropriate if you're stuck for ideas.
You could include vows to your children within or separate to your vows to each other.
Older children may be able to make vows to the two of you.
Involve them in a hand fasting ceremony - a different ribbon or cord for each child.
Involve them in a unity ceremony - a sand ceremony, different coloured pebbles being dropped into a jar, different coloured lego... so many possibilities.
Have them bubble the ceremony. Give the children some bubble blowing gear and let them loose. They could walk down the aisle with you, blowing their bubbles, or just blow from their seats (or wherever they are) before, during or after the ceremony.