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Either will do. OCI is nice since it's for life, but it can take up to 6 months to process. Regular travel visa is much cheaper and you usually get it within a week or so. NOTE: You should surrender the child's Indian passport after registration goes through and don't attempt to use it out of 'convenience'. They are rather picky about that right now.
It seems that only the form got updated in January 2018, due to an out-of-date question. Therefore the 2017 guidance should still be valid.
Application to register child under 18 as British citizen: form MN1 wrote:Published 1 September 2013
Last updated 24 January 2018 - hide all updates
24 January 2018
Replaced application form as it contained an out of date question.
13 January 2017
Updated form and guidance.