There are 2 parts to this. The broker where the account is held and then the investments you choose.
I have my (adult) stocks and shares ISA with Scottish Widows. As long as you stick to ETFs, shares or gilts rather than funds fees are low and I find the interface easy to use. There are other low cost platforms (eg Trading212) which came along after I had opened my account, but I haven't bothered to transfer. I don't know what any of these platforms are like for junior ISAs.
Moneyvator have a useful table comparing different brokers: https://monevator.com/compare-uk-cheapest-online-brokers/
As he already used the maximum jISA allowance for this year (he doesn't get the adult allowance on top of the child allowance) are you thinking of transferrrng from the cash jISA to a stocks and shares jISA? If so remember to go through the transfer process rather than just withdrawing it and trying to pay in elsewhere.
I personally wouldn't invest in stocks & shares outside a sheltered account, but I'm not any sort of financial advisor, but I think not having to worry about Capital Gains Tax on (hopefully) future gains makes life simpler.
Moneysaving expert has a list of child savings accounts where you could shelter the £5000 til next year https://www.moneysavingexpert.com/savings/child-savings-tax-free/
Another idea would be to invest some in a junior SIPP. Obviously very long term, but you can invest up to £2,880 and the government will top it up by 20% - so free money!
As to what to actually invest in, as I said I'm not a financial advisor or any sort of money expert, but a low cost world tacker would be my choice. Vanguard have just launched a all-cap global tracker ETF. The accumulating version (ie where the dividends are automatically invested) is called Vanguard FTSE Global All-Cap ETF Acc - the ticker is VALL.
Just to say again, I'm no expert and you should always do your own research. Moneysaving Expert and Moneyvator are good places to start.