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In this exact situation (2yr old kid, Fidelity) I picked HSBC FTSE All-World Index Fund C GB00BMJJJF91 per here: https://monevator.com/best-global-tracker-fund/
(Yes, I only really take advice from Monevator but it seems solid to me)
Note that opening a SIPP for your little one is also a good option. You put in up to £2880 per year and gov top it up to £3600. Invest as you see fit.
What's a good fund/ETF pick for a little one's ISA with Fidelity, which won't be touched for at least 18 years from now?