First of all, you need to o change habit of giving milk bottle during sleep... Because baby teeth are very sensitive and it gets decayed very soon so after giving milk you let baby wash his mouth so that the milk particle sticking upon the teeth surface should not get decompose and decay the tooth surface.. You let the baby drink some water to wash it off ..

Visit dentist and get tooth filling done at earliest. Toothpaste you can use kidodent pea size toothpaste should be used. If child is not able to brush you can help in brushing. Brush twice a day. Ask child to rinse mouth after eating everytime.

Its just a start of cavity in front two teeth. Get it filled at the earliest before it reaches the nerve. Hand brushing is more than enough and electric brushes are not required at this age. Use any non fluoridated tooth paste like pigeon company or kidodent. You may further your dentist to prescribe one till the age of 6 years. Brushing once is not enough. You have to encourage child to do night brushing as that's is very important. Find out innovative and interesting  ways to motivate your child for night brushing too. Once or twice if they don't do thats OK but once it becomes a.habit for your child they themselves don't skip brushing.make it an enjoyable time for them. You can try musical brushes or different cartoon character brushes for them.