I'm not a financial advisor, but I use these passive ETFs.
LGGG - Legal & General Global Accumulation Fund
Ongoing charge = 0.1%
https://fundcentres.landg.com/srp/lit/76OdV8/Fact-Sheet_LG-Global-Equity-UCITS-ETF-Global-Equity-ETF-USD-Acc_28-02-2025.pdf
Legal & General also have an S&P500 accumulation tracker (LGUG), which I think is the best available (0.05% ongoing charges), but it isn't very popular ('has low liquidity') so it's maybe not the best for your requirements.
VUAG is the Vanguard equivalent with an ongoing charge of 0.07%.
I, personally, would probably just put it all into LGGG as a monthly contribution to a Junior SIPP and Stocks & Shares ISA. The purchase price will even out over time.
Interactive Investor don't have any dealing charges for regular investments, but they do have a monthly platform fee that rises to £12 a month (£144 a year) once you've got £50k invested.
iweb have no platform fees, but charge £5 per trade, so would be £60 per year if you had a regular monthly purchase. If you stopped trading, you'd then pay nothing. I use them for my ISA.
Fidelity don't have a platform charge for Junior SIPP accounts and it is £1.50 per trade, so potentially £18 a year in fees for monthly investments into a single fund, such as LGGG.