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I cannot see that a payment of £60,000 could be seen as anything other than a lease premium.
As the length of the lease will be less than 50 years, part of the lease premium of £60,000 will be taxed as income and part as a capital gain. It's complicated by the fact that your brother has a 50% interest in the property but the principles are the same.
See Example 2 in HS292 here. In order to determine how much of the premium is chargeable to income tax and how much is chargeable to capital gains tax, you will need to know the CGT cost of the property and the value of the "freehold reversion" of the 50% of the property that your children own which will be subject to the 30 year lease. If you can find that out, then you can either follow Example 2 yourself to arrive at the figures you need or I can do it for you.
I hope this helps but let me know if you have any further questions.