First, the UN does not have a standing military. UN peacekeeping troops are first and foremost members of their own nation's military that are then seconded to work for the UN.
The following is from the UN's website:
https://peacekeeping.un.org/en/military
"The UN can only deploy military personnel when there is a UN Security Council resolution authorizing them to do so. The Security Council will say how many military personnel are required, and UN Headquarters will liaise with the Member States to identify personnel and deploy them. This can take time – often more than six months from the date of the resolution to get boots and equipment on the ground."
Russia is one of the five member states with a permanent presence on the UN Security Council. The other four permanent members of the UN Security Council are the US, France, Great Britain and China. The five permanent members can unilaterally veto a UN Security Council resolution.
Even if Russia didn't veto a Security Council resolution to send UN Peacekeepers, it would still take time to liase with member states and get boots on the ground.
The article found here on the UN's website (
https://news.un.org/en/story/2022/03/1113722) dated March 10 details the humanitarian aid the UN has provided.