1 It is dad's birthday today. - ok
2 Mike and Ted are making soup to surprise their mum. - ok
3 They are making Whities - не понимаю, но кажись всё ок - в тему
4 Their cake looks like an old boot - ок
5 Mum thinks this is a terrible surprise. - ок
6 Mike and Ted must clean the bathroom before the guests arrive.- ок
то есть грамматически все ок. может с фактами что-то не так?