We have a small pond on our allotment and my children (6 and 3.5) have great fun around it and complain when it's time to go home, they don't want to leave the pond. But I still have to keep them in my sight. With an 18 month old I would be very cautious indeed and wouldn't consider a pond (no matter how small) unless it was very well covered, you can get that steel square grid (I think they use it in building as reinforcing or something), the squares are big enough to let frogs through but small enough to keep children out. Having a wildlife pond is lovely and I highly recommend it but be very cautious with little children.
