The children get free children's passes but you need to pay the same reservation fee for the children as for yourself. 

The reservation fee for the Eurostar to get to mainland Europe is 30 EUR in 2nd class per person.

To manage London - Barcelona in one day you need to take the TGV from Paris. The reservation fee is 34 EUR per person.

From Barcelona to San Sebastian you need a reservation from 8.5 EUR per person . This reservation is tricky to make.

It takes between about 15 and 26 hours to travel between San Sebastian and Brive so that will be hard to manage in one day and the reservation free is from 44.5 EUR per person.

The reservation fee for the travel back to the UK is from 44 EUR per person.

This is a challenging itinerary, especially with 2 young children. London to Barcelona in a day is difficult as there are only 2 trains a day from Paris to Barcelona. Even then it will take about 14 hours with a station change in Paris. Meaning a late arrival in Barcelona.
Barcelona to San Sebastian is straightforward but needs several hours and a change (Some direct trains but only showing one in the afternoon.
San Sebastian to Brive is a convoluted journey and probably needs a return to Barcelona.

Brive to London is relatively straightforward but with station change in Paris.

Many of these trains have mandatory expensive reservations and all passengers have to pay (AFIK the price is the same for all including over 5s (Under 5s sit on knees). So travel for children is free but reservations cost standard prices.

Eurostar is 30 euro per passenger each way, Par - BCN is 38 euro per passenger and domestic TGVs  between 10 and 20 euro per passenger.Some Spanish trains also need reservations.

You can get the total price for your passes by using the selection box tool, but yes one adult with 2 children 4 - 11 costs 258 euros. For 3 passengers this could easily add up to 400 or 500 euro. 

I would though suggest you at least get the 5 day pass and split the San Sebastian to Brive trip. It isnot much more expensive. I

@Yorkie

Children age 4 and older need their own pass and reservations with Interrail. 

I stand corrected - I assumed that from the OP she realised that free passes were needed. 

You'll need to add seat reservations sure but all these trains are expensive anyway so compare the prices between a pass and regular tickets

- Barcelona - San Sebastian reservation can be hard to get but I don't think seats will sell out until you arrive in Spain.

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index2.html?postid=18419#post18419

- San Sebastian - Brive does not take 15-26h.

- You need to take the Euskotren (2,70€, not included in the pass) : runs every 30 min between San Sebastian and Hendaye (French border)

- TER Hendaye - Bordeaux St-Jean 15:04 - 17:41

- TER Bordeaux St-Jean - Brive 17:58 - 20:24

All reservation-free

This is one good connection, leaving San Sebastian after midday.

Let us know for any other questions

seat61.com will be very helpful

https://www.seat61.com/interrail-and-eurail-reservations.htm

Thanks so much everyone. How do you know if a journey needs reservation or not? And how do you find out the price for the reservation? I might miss San Sebastián and go from Barcelona to Brive, I know that’s a pretty straight forward trip (I did it last summer with the kids)

London to San Sebastian is also a lot faster if you go via Hendaye. But maybe you want to visit Barcelona as well.

The daily direct train Barcelona-San Sebastian has a €6.50 reservation.

An overview over reservation prices can be found here:

https://www.interrail.eu/en/plan-your-trip/reservations/reservation-fees

Some are a bit outdated but you'll get a rough idea.

So in summary all the routes are covered with the Pass but high-speed trains in Spain and France (+ Eurostar) need reservations.

- Eurostar 3 x 30€

- Paris - Barcelona 3 x 37€

- Barcelona - San Sebastian 3 x 6,50€

- San Sebastian - Hendaye 3 x 2,70€ (maybe children fare)

- Brive - Paris 3 x 10€

- Paris-Montparnasse - Paris-Nord about 2€ RER

- Paris - London 3 x 30€

Total : 220£ pass + about 340€ reservations

Another option to reach Brive would be through the Pyrenees, the only reservation needed would be Toulouse - Brive on IC trains (3 x 10€) :

- Barcelona - La Tour de Carol

- La Tour de Carol - Toulouse

- Toulouse - Brive

If Brive is not mandatory, Bordeaux is a beautiful city and easier to reach : San Sebastian - Hendaye - Bordeaux - Paris

Note that the 4 days/1 month pass ist currently £227 on interrail.eu and £214 on myinterrail.co.uk, so that's an easy saving.

Seatreservation child. How do I make it on your sight fore a child 2 years old

It's best to create a separate post. For questions about making reservations, please include route, date and departure time.

Do we make s seperat post with birthday and passport number?

Of course not, please don't share personal details.