I have had a thermal hottie for about 5 years. It is fab - it has material in it and just a little bit of water so it isn't as dangerous as pouring boiling water in a normal bottle and doesn't make so much mess if it leaks. It goes in the microwave for about 2 mins 30 and doesn't smell like the wheat filled ones. Google 'thermal hottie' and you'll find them on the net.
I bought one each for my grandaughters age 4 and 5. they've got animal fur covers, one is a Tiger and the other is a Zebra. In a Linen shop in Southend £4.99 each.