Call nanny and ask her to call you/or visit if local at a convenient time as an allagation has been made against them and you need to know the situation from their side.
Will you suspend them on full pay pending investigation? If they work Thursday and it subsequently goes to tribunal then I would wonder what the tribunal would feel about you continuing to have them care for your child. By having them work thursday I feel it may be harder for you to claim you felt your child was unsafe being with nanny.
We don't know the geographics of things. With luck you do, so look at what risks were taken. Why were they at a place near the canal lock? Did nanny know it was there, was the risk acceptable - heavy gate, your dd had to deliberately open that.
Yes nanny should have been keeping a good eye on your DD but your DD may like having some independence. It's a fine line between wrapping them in cotton wool and giving them some freedom. Look at the risks involved - did nanny deliberately put your DD at risk, or did it happen by accident?
Why did DD run? Would she have done that for you? Will she do it again? Your DD is not innocent by the sounds of it, she ran, she deliberately opened a gate. Why?
If this is the last straw, then dismiss with notice if you so wish to do so. Dismissal without notice may be more tricky, as the nanny could claim that it was an accident, that your dd ran away. They didn't deliberately endanger your dd by taking them to the lock and letting her sit on the wall.