Spanish railways are generally a pain in the lower backside. They made making reservations difficult for others, including for Eurail. Spanish domestic trains can currently not be booked via Eurail.

For the remaining possibilities, see this post. As to what RENFE said: indeed you can make a pre-reservation by phone but you'll have to collect it within 72 hours. I think this will be your best chance of securing a reservation, unless you will be in Germany or Switzerland before you go to Spain. The good thing is that reservations in Spanish domestic trains are not quota-controlled, so you can book seats until the train is full.

Thats it: a fone-REServ (very common to do in ESpana) will be held for you max 72 hrs-so you have to bypass a counter and collect/print it out there in that limit.

Its FR eve so thats a busy time as such. In ESpana its always very wise to have plan B and at least to look at what other options there are for just in case…… This is by far not the only train running Bcn-Vlc that day. There is still 1 regional=NO reserv needed nor even possible, but it starts from Bcn like 9-10 am.

Thank you both so much for the great info! I guess I’ll phone ahead a few days before and hopefully they will be able to accommodate. All of the other train options seem like they take FOREVER to go from BCN to Valencia.I think there is a later fast train as well but I didn’t want to arrive in Valencia too late as we’re traveling with 2 small children and we’ve never been there before and will have no idea where we are! Anyhow, thanks again for the advice!