When my bucklings were 10 weeks old they would not drink from the stock tank. The only way I knew for sure they were getting water and learned to like the water as opposed to the the milk replacer was to give them a midday bottle of warm water.
At 5 weeks they should be down to a bottle of milk 2 times a day (yes?) - with a momma goat they would already be nibbling hay and grass or browse.
I would do the following -
Put them out during the longest part of the day and bring them in at night for a week to two weeks to slowly wean them...