walikum as salam,
For a one-off basis lump sum, it’s probably best to open an account with AJ bell and invest in the HSBC Islamic fund “BC GBP” version directly as this would be cheaper - AJ bell charge 0.25% as platform fee whereas red rose charges 1.57% so it makes sense to stick with AJ bell for your scenario. (Fund is same in both cases so should have similar charges).
Also, I am not sure if red rose or others are only offering regular investment options and not one lump sum option. For a child account, they are probably highlighting the option of regular investing as that is a very good strategy over time but I doubt that any provider restricts anyone planning to just do a one-off payment. Below is the extract from the red rose website - under the main product features and to me it suggests that there is no restriction for a one-off contribution basis. The best thing is to call them and clarify if you are keen to go with the red rose.
“There is no set premium – you can pay in regularly by monthly direct debit of £10 or more and/or invest lump sums at any time up to the annual Junior ISA limit”
Also, If you put the child in the search bar, you can find a few threads discussing the topic. Below are the links for some posts on child investment that you may find useful.
Hope this helps
Regards
Zeeshan