Between Fonteblanda (Talamone) and Albinia there is a pretty well known long stretch of sandy beach that could be good for small children. The road to Rome airport coming from Grosseto passes through there. It’s called Spiaggia dell’Osa (Osa Beach), from the name of the Osa River whose mouth is there.
I wouldn’t stay too close to Orbetello. It sits in the middle of a lagoon and it’s too infested with mosquitoes for my taste.
Along the aforementioned beach there are lots of nice campgrounds but I imagine they will want a hotel.
I found one on that beach, check it out.
https://maps.app.goo.gl/YkUehAMUx5sc24Gn9?g_st=ic
There are others nearby, like the Argentario Osa Resort or the Fontermosa hotel. Also more accommodations are in Talamone.
They will probably need to spend the night of the 31st at the Rome airport hotel however. It’s more than 2 hours from that beach to Rome and you need to check in at least 3 hours prior to departure. Italy is running out of jet fuel in about 3 weeks if the Strait of Hormuz doesn’t open, so they may have time to enjoy the beach for a lot longer. It’s not a bad part of the world to be stuck.
PS. Make sure they rent baby car seats. Mandatory for under 12 y.o.