If you’re just doing return trips to these cities, a rail pass is unlikely to be the best value.
A return ticket on the Eurostar starts at £78, or even £58 in a sale, although you won’t find these prices on peak Frida/Sunday trains.
With a pass, you’d have to buy seat reservations on Eurostar costing €30 each way, on top of the cost of the pass.
Additionally, if you’re a UK resident, you’d be limited to only 2 days travelling in the UK (and the Eurostar counts as UK) per pass. (You should check here to see if you need a Eurail or Interrail pass. If you can get a Eurail pass, then the 2 day limit won’t apply to you.)
If the children are under 12, they get a free railpass when you buy an adult pass, but they still have to pay the €30 seat reservation fee.
If I were you, I’d look at the low fare finder on the Eurostar website, and see what prices you get for the weekends you’re looking at.
https://www.eurostar.com/uk-en/find-lowest-fares
Compare this with the cost of the passes + seat reservations. And bear in mind that the seat reservations for passholders are limited availability - you might not find them on the peak Friday and Sunday trains.
As you seem to live in the UK, you can only travel on a maximum of 2 travel days in the UK. This includes travels with the Eurostar.