This is clearly a difficult question to answer as it depends upon the reasons as to why the child or children do not want to see their Dad. Children should be encouraged by their Mother to see their Dad if it is a safe and loving environment.
The starting point is that children should grow up knowing both their parents. If there is an issue as to why they do not wish to go to see their Dad then that needs to be explored and resolved. Sometimes it has to be resolved using professionals whether that be medical professionals or legal professionals.
If you are a Dad whose children are indicating that they do not wish to see you, it is important to act quickly as any delay is likely to be prejudicial to you
It is important that children are not ignored. When children reach a certain age and understanding their wishes and feelings will be taken into account. The legislation that deals with children matters is the Children Act 1989 and the Courts paramount consideration is always the child’s welfare. The Court also considers the welfare checklist (a set list of factors) in deciding what is best for the child (see article here for further information on welfare checklist).
You should try and resolve any dispute regarding contact with each other first. You could use mediation to assist you in doing this which could include the children if they are old enough and understand the situation. You can find mediators near you here.
If you cannot resolve the matters then Court proceedings may have to be issued but this should only be seen as a last resort. Many people believe that going to Court will resolve their problems but in my experience whilst the Court can make Orders in respect of arrangements for children, those issues still need to be resolved so that they do not occur again.
If you are a Dad whose children are indicating that they do not wish to see you, it is important to act quickly as any delay is likely to be prejudicial to you. If Court proceedings have to be issued then those proceedings will not be resolved overnight they can take months to resolve and important time will be lost if you do not take action.
There are many solicitors out there that will be able to assist Dads in this situation usually by offering a free initial appointment. My advice is to look for a Resolution Member as they adhere to a code of conduct so that your matter can be dealt with constructively and in a non-confrontational way. This will assist in avoiding the situation getting worse.
Signposting to additional support and relevant organisations can be found in this link.
Postscript. Issues arising out of divorce or separation can often prove stressful for the couple and other family members. This article written by Dylan Watkins GP offers a useful overview for those who may be struggling.