Couple of things to consider.
You don't get the family card with the regional pass, so the two kids will each need a 30 CHF Child Regional Pass.
You will not be able to take the EC train all the way to Spiez as it is not valid on the EC route. Instead, you will have to get off at Brig and take the "Loetschberger" train to Spiez, which is slower, although more scenic. In that case you could just buy tickets to Brig, at the Trenitalia site if it costs less, for you and the kids.
If you are getting a 4-day B.O. pass it is only 9 CHF less than the 4-day Swiss Pass and you have to get 30 CHF Child passes for each of the kids. If you are getting an 8-day B.O. ppass, it is only 26 CHF less than the 8-day Swiss Pass and you again have to get Child passes. So it only might make sense if you are getting a 6-day B.O. pass for which there is no equivalent Swiss Pass. And then where are you traveling afterward?