What type of visa do I need if I want to travel with my partner (unmarried) to the UK to visit his children

I think you're looking for a Standard Visitor Visa. It can be used:

• for leisure, for example on holiday or to see your family and friends

This visa can be issued for 6 months.

The documents you have to provide are:

• the dates you’re planning to travel to the UK
• details of where you’ll be staying during your visit
• how much you think your trip will cost
• your current home address and how long you’ve lived there
• your parents’ names and dates of birth
• how much you earn in a year

You might also need:

• details of your travel history for the past 10 years (as shown in your passport)
• your employer’s address and telephone number
• your partner’s name, date of birth, and passport number
• the name and address of anyone paying for your trip
• the name, address and passport number of any family members you have in the UK
• details of any criminal, civil or immigration offences you have committed

Here is a list of all supporting documents you can choose to provide.

You also need to prove:

• you’ll leave the UK at the end of your visit
• you’re able to support yourself and any dependents for the duration of your trip
• you’re able to pay for your return or onward journey and any other costs relating to your visit
• you have proof of any business or other activities you want to do in the UK, as allowed by the Visitor Rules