It’s valid on the Elizabeth Line to Heathrow, but not the tube or Heathrow Express.
For whether it’s good value you have to do the maths. Compare the price of the pass + compulsory reservations, with the cost of individual tickets.
I think it’s unlikely a pass would be good value for you, but if your children are under 12, it’s possible, only if the cash prices on Eurostar are high.
You can check Eurostar prices on their website. They have a special destination of ‘any Belgian Station’ which would save you having to buy a separate ticket to Bruges, if your onward connection is within 24 hours. In any case, the ticket won’t be expensive if you have to buy it.