Low light lens for my Lumix G7 or buy new camera

I would stay with MFT, not going for RX100, as it by itself doesn't solve Your problem . Fast glass, but also a smaller sensor. Go for fast primes and flash.

25/1.7, 30/1.4 are good options, I would add Oly 17/1.8 and maybe Sigma 16/1.4 to the set. Nothing longer (not good indoors), nothing wider (severe body distortions when taking portraits from a close distance). Note, that Sigmas are big & heavy = not good for travel. Oly 17/1.8 and Pana 25/1.7 seem most universal. Pana 20/1.7 is also excellent, but its AF struggles sometimes in low light.

Note yet another thing: Your target for indoor photo are moving people, including children. Very difficult target. Even fast prime may be not enough. There's only one solution -add light indoor by using flash. But not just built in flash straight into the eyes. Use small  & cheap external unit  with posibility to bounce off the ceiling. You don't need anything else.

Quite different story is low light photo of non moving targets. It may be still Your family when... still (for example at the table or posed portraits). Fast prime lens without flash may suffice.

Another case is photographing just interiors (in tourism: historic buildings or shrines/churches, museums etc.). Using flash is here difficult or impossible: too big spaces, flashing is banned or disturbing  to others etc. Tripod can be  troublesome, too.

Here fast primes can do miracles, but there's one problem with Your current set: lack of internal stabilisation (IBIS) in G7 body. Many fast primes in MFT , including all discussed above have no stabilisation. They will suffer at low light, static scenes when paired with camera body without IBIS. Just loosing some of extra capabilities by it. Fortunately, Panasonic currently does some cameras with IBIS, You don't need to go Oly   Mostly some (not all) bodies from GX series. Check carefully when choosing one, because sometimes there're mistakes in specifications at web resources.

In summary, I recommend You the the following path:

0) Keep 14-140; it's excellent travel/multipurpose lens

1) Buy a flash and learn how to use it properly; let it be unit compatible with Pana/Oly/MFT, not "universal" one as You loose TTL function otherwise; beware of old analogue time flashes as some of them may be dangerous to digital camera (too high voltage on foot connections); the cheapest modern Chinesee flash with tilted reflector, dedicated for MFT should be O.K., they are really cheap & good enough; oryginal Pana/Oly units are  prohibitive expensive, completely without sense...

2) Buy a fast prime, from the list discussed above

3) Possibly in future exchange Your camera body for one with IBIS

My own experience account: usually travel with Oly cam + 14-150 zoom and 17/1.8 prime. Added lately 12/2.0 and a small flash unit, unfortunately without bouncing ( try to use home-built large diffuser).

Regards,

-J.