How do I put on weight?

I am 28 yrs old, 5'5ft height, weight 46kg. I eat 3 meals a day and try to have a snack in between meals (often fruit, sometimes crisps / biscuits). Breakfast is usually a bowl of cereal (eg. bran, muesli, weetabix), lunch (sandwich), dinner (meat and vegetables). I have been trying to gain weight for ages. The most I've weighed is 49kg. What are good snacks to help weight gain (should they be high calorie snacks?)?? I spend my day running after 2 toddlers so need easy 'on-the-go' snacks. Any suggestions on what / how I should eat to help me gain some weight would be greatly appreciated! Thanks!