If you wish to assist the people of Ukraine with humanitarian aid, the International Red Cross has set up a Ukraine appeal. They are working with the Ukrainian Red Cross to provide food, water, clothing, shelter and medical care to displaced persons. You can see their Ukraine appeal page at https://www.icrc.org/en/donate/ukraine
If you specifically wish to help children in Ukraine, you could direct your donation to UNICEF, who are also running a Ukraine relief campaign. More information on their work and how to donate can be found at https://www.unicef.org.uk/donate/donate-now-to-protect-children-in-ukraine/
Alternatively, the International Rescue Committee is working with Ukrainian refugees who have fled to Poland. If you want to help IRC with their work supporting Ukrainians who have crossed the border into Poland, please donate at https://www.rescue.org/topic/ukraine-crisis
If you wish to give to Ukrainian organisations specifically, the Association of Ukrainians in Great Britain is sponsoring an appeal along with a number of Ukrainian churches and civic groups based here in the UK. They are working with accredited charities in Ukraine to provide emergency aid to those in need in the country. You can see their GoFundMe page at https://www.augb.co.uk/
