You could use
– a large, chunky, church candle which will last a long time
– a long, thin candle in a candle stick which will raise the flame away from the table but be easier to knock over than…
– a tea light, which you’ll need to replace quite often but you could easily have one per child if everyone wants a go at lighting it (with adult assistance of course!)
You could make up your own prayer, or use one based on a verse in the Bible about light. We use this one based on a traditional Jewish prayer and some Bible verses (John 9:5, John 8:12) as part of our weekly Shabbat meal and you’re welcome to borrow and adapt it:
We bless You, Lord our God,
who has given us Jesus, light of the world.
Thank you that whoever follows You
will not walk in darkness
but have the light of life.