Depends on hotel policy and staff at the time. When we last stayed a couple of years ago, the hotel made a song and dance about allowing our daughter (who was 3 at the time) come in to the lounge with us on one visit and were perfectly fine the next day. Having two kids and them both being adult(ish) age may give you more of a challenge. Worst case is that they ask you to pay.

However, despite all of the above, even with the roadmap in England to opening lots of stuff up by June, there is no guarantee that the lounge will even be open at the time of your visit, although the chances have now probably increased a bit after the announcement yesterday.