Try getting some baby porridge or egg food abs make it up with puppy milk or soy baby formula. offer them a sloppy bowl of this every day. If you also but some milk substitute in a water bottle as well as another bottle with water in you should see if they drink it. Go for small mouse sized bottles for now. Also keep dry food around midday of the time. They should be able to eat it so don't over do the wet food, give them maybe a desert spoon full and see how they do, maybe offering another later on. You can start adding more solid wet foods as they get on like cooked rice and egg, boiled and scrambled egg, cooked fish etc. aim for then eating half wet half dry food abs keep the protien content up.
At this age they are about to go through what i fondly call the chicken phase. This is when they turn from plump looking babies into straggly long looking babies. They often look a little scrawny as they need to catch up with thete sudden growth but if they start feeling boney then they aren't getting enough. thew tails should also be nice and round and full.
If they don't use the water bottles you can show then with your finger getting them to luck fluid of your finger abs getting closer to the bottle until they luck that instead. They normally learn from there mum though
