I have a small concrete child holding a birdbath and it has a hairline crack across the bottom

I noticed that it was leaking water from the underneath and that's when I saw a hairline crack. I'm worried that anything temporary will crack even more when Winter arrives and the water freezes. I suppose I could try the stuff that stops leaks in gutters but thought Hypertufa might be stronger.

You can pick up a tube of silicon caulking at the DIY store (Home Depot or Lowes in the US). It's used for building fishtanks so you know it will create a watertight seal. I like clear because you won't even see it when it dries.

Thanks. I shall have a go at that Bathgate.

I was going to suggest bathroom/silicon sealant, as Paul suggests too. you can get a clear one used in fish tanks and then a range of colours such as grey/black brown.

You'll probably find 100 other uses for it around the house.

We've got enough leaks in the house, so it might come in useful!