Unless you're willing to take some silly diversion, which wouldn't be practical for a family day trip, the only advice we can really give is to avoid HS1 - which will add anything up to an hour to your journey, for the sake of saving ~10%.
Sorry no its just for a single and it's rye near Hastings , I'm not aware of routes on hs1 ? --- old post above --- --- new post below ---
I had planned a group travel card from slough then east Croydon to rye .
or singles Windsor riverside to Clapham junction and then Clapham junction to rye .
Here is an alternative option that may be worth a look:
Slough - London Paddington - London Bridge - Redhill
--- split ---
Redhill - Tonbridge - Ashford/Hastings - Rye
This saves almost £15 on the HS1 fare, and £8 on the non-HS1 fare. However, NRE is quoting a journey time of roughly 4½ hours, even though direct services operate on this route between London Bridge and Tonbridge (admittedly only once an hour though)...
In other words, stick with the £63 non-HS1 option. At least that way you can get a direct service through to Ashford from Charing Cross and change there.
Yes, it's too late to add people for tomorrow. More importantly, if you are using yours then you can't go via Ashford as the whole journey must be made on Southern services and Tonbridge to Ashford is only Southeastern.