Guidance on long term savings - are stocks best?

Hi all, I was just wondering if I could get some advice on what I’m best doing. I’ve been saving heavily for a long time but that was to buy my first home so it’s been very focused with money going into accounts like my LISA and not looking in the distant future. Due to this and my very limited knowledge, I know basically nothing about stocks and shares other than the actual savings amount can go down as well as up (this is what has always put me off finding out more).

The reason I’m asking is because I want to save properly for my nieces and get the best for them. My sister and her partner are barely making ends meet and my sister always has been useless at saving, so they won’t be putting anything aside to help my nieces when they’re older which is why I want to do something and get the best I can for them. It also means I’m unable to open a JISA or premium bonds or anything in their names as I don’t want my sister and her partner to know about them (they’d try to take some promising to pay it back but wouldn’t or would try and take some of it from my nieces once they’re old enough to access it). I also don’t want my nieces to have access too early and then spend it all on rubbish like my sister did when my grandad did the same for us at 18.

I have two step-nieces who are currently 5 and 3 and one biological niece who is currently 1. I’ve got a few savings accounts where I put £100 in for the day they were born and then doing £5 each a week since (I’ve backdated this for my step-nieces so they don’t get less just because I didn’t know them from birth) and plan to keep doing this until their 21st birthdays.

At the moment they’re in old savings accounts so the interest rates are appalling. I have a current account with Nationwide so have access to the savings account with 6.5% interest, but you can only have one of those and I like to keep the three accounts separate so it’s fair on the three girls when it comes time to give it to them. My understanding of this option would be that I’d keep the 3 pots separate but put £15 a week into the account with the 6.5% then at the end of the year, split the whole amount by 3 to put into their individual pots. However, this means the main bulk of the money isn’t getting the 6.5% interest so I don’t think this will be the best option.

So my question is, would I be best putting this money into stocks considering it’s going to be more than 15 years before the eldest turns 21? Would I be able to open an account to have the three separate pots for them? Over that amount of time, what are the chances of me getting out less than I put in? I’ve seen that there are fees on the stocks accounts that I wouldn’t need to manage myself (I wouldn’t have a clue what stocks to put it in and would forget to change them etc). Are these going to make it not worth it for the amount of money I’m talking about?

I’ve read the money saving expert page and saw it recommended the Wealthify managed stocks account. Does anyone have any experience with this? Is that a good one for me to use for what I want to do? Where it says a minimum deposit of £1000, can I have three pots in one account or will I need to open 3 accounts with at least £1000 in each (which is fine for the older 2 but I’d have to top up the youngest one and stop contributing to it when she’s about 19 1/2 rather than when she’s 21 like I will for the older two)?

Apologies for the ramblings and thank you in advance for any help with this. I’m completely financially illiterate when it comes to utilising interest rates etc. Also, if anyone has any better ideas for what I should do, please let me know. I want something though that I can just set up a standing order from my current account to pay into each week then forget about until the girls are 21.

I did have a look at the investing for your children wiki page but a lot of it didn’t relate to what I need because I don’t want tell their parents about it so can’t get the important documents to be able to do what’s suggested and I also need help on how to do stocks correctly.

Thank you!