If the tap handle is in line with the tap, you could try winding a strong elastic band around the whole thing. That is easy for an adult to get on and off but hard for small hands.
That's what I used to do with double cupboard doors - a thick elastic band around the two handles - you can only open it a wee bit then. You need eyes in the back of your head with a toddler!
I am sure I am telling you something you already know, but make sure too that there are no other nasty things hanging around the lottie like chemicals or sharp tools, they can be fun but also a dangerous place for a very small child. My 6 year old was running around with a seedling planter the other day, good thing to fatally stab yourself with.