Your children can’t contest your will just because they think it’s unfair.
They (and other family members, and anyone financially dependent on you) could claim in court that your will does not make ‘reasonable financial provision’ for them. If they succeed, the court can effectively rewrite your will so that they do get reasonable provision.
However, it is extremely unusual for adult, independent children to succeed in a claim for reasonable financial provision. A side letter explaining why you are leaving everything to your partner — for example, because your children are grown-up and financially independent — should significantly reduce the risk.
If you are elderly, your children might try to show you didn’t have mental capacity when you made your will — you did not understand what you were doing. If there is any risk of this you may want to get a certificate from your doctor confirming that you are mentally competent when you make your will.
You can discourage your children from challenging your will by leaving them something in the will — but at the same time including a clause saying that anyone who challenges it will be disinherited.
You may want to discuss your intentions with your children to help them understand what you are doing and your reasons. If you feel they are likely to challenge your will, you should let your solicitor know.