The answer is that you probably do. If their father has Parental Responsibility for the children (see another article "Can I stop my ex-partner taking our children abroad on holiday?" for more information about what Parental Responsibility is) then you will need to seek his permission to take them out of the country even if this was a day trip. The only exception to that is if you have a Residence Order in your favour and if you do then you are allowed to take them out of the country for up to a month without his permission.
My advice is you ask for this permission as soon as possible. If he says no or if he will not commit to this then you may need to consider applying to Court to get a Specific Issue Order allowing you to take them. As the Court is very busy it may take some weeks to get a hearing date and even then matters may not be sorted out at the first hearing and it may be that a further hearing is needed, or statements need to be filed or even that the Court welfare officer (CAFCASS Officer) needs to make a report.