I believe the Group 3 Return ticket is only available in Anytime form so there's no risk that you'd mistakenly get an Off-Peak version, but you should check that the price you're charged is £75.80. You can also buy the ticket online from your favourite website, selecting the specific trains you intend to take (make sure the price is correct, and that you're not getting a split ticket or an advance ticket), but the ticket is of course valid on any train. (There is no period return for a single person.)
However, I don't think that's the best way to do things.
Looking at the
Britrail website, the London Plus pass is shown as only having 3,4,8-day flexi passes. I can only see an option for 2 days for the Great Britain pass. So the options (for a German, as quoted by
traintickets.com) (EDIT: found some GBP prices at
internationalrail.com) seem to be:
Great Britain 2 days: EUR 131
London Plus 3 days: EUR 141 (£116)
London Plus 4 days: EUR 172 (£142)
If this is correct then it just costs your visitors EUR 41 extra to add 2 days to cover the Stansted travel.
Your travel is fully covered by the combination
Leicester - Stansted Airport Off-peak Return Not via London: £50.00
Stansted Airport - Leicester Super Off-peak Return Any Permitted: £69.90 (Appears to be valid during peak between Stansted and London - peak restrictions only apply on the main leg between London and Leicester)
Total: £119.90
Using the group ticket for Stansted would cost
Leicester - Stansted Airport Off-peak Return Not via London: £50.00
London Terminals - Leicester Super Off-peak Return: £54.10
Stansted Airport - London Terminals Anytime Group 3 (Return): £75.80
Total: £179.90
So this costs £60 extra and saves EUR 41 - not a good deal.
Another aspect to consider is the risk that you unexpectedly become unable to undertake the second trip. If you have used a bunch of outbound legs on the first trip then you can't refund the return legs, and your visitors would not be able to use the group return ticket without you so would have to pay again for new tickets.
So it's better to keep your tickets separate, and further if you're willing to go through the faff then you should get Haywain's originally suggested version (LEI-SSD Not Via London, changing the return leg to Any Permitted for the first trip, then get the same ticket again but changing the outbound leg to Any Permitted for the second trip). This costs the same (up to rounding) as the two separate LEI<->SSD tickets, one with each route, but allows the second trip to be refunded if you become incapacitated.
Even if your visitors get a 2-day pass and need to pay separately for Stansted Travel, standard Singles for an Adult and a Child would cost (£21+£10.50) off-peak + (£23+£11.50) peak = £66 (with potential additional savings from any connecting leg to/from Liverpool Street being combined with the Stansted Express fare, which happens automatically if the adult uses contactless), so the total would be at most £6 more than using the Group 3 ticket for Stansted.