You can probably improve those speeds a bit, but depends on the device. For 2.4 GHz you might want to try changing the channel manually (choosing channel 1, 6 or 11) and see if that helps - and turn off automatic optimisation, that's a feature that causes more trouble than its worth and may explain the kids' connection woes. Typically you won't see much better than 80 Mbps on a 2.4 GHz connection. For 5 GHz you may be able to improve speeds by increasing the channel width to 20/40/80, and with that you should (if the device allows it) see speeds up to 350 Mbps and maybe higher.
As for wireless strength, VM's hubs have always been poor wifi routers. Most of the regulars here (including me) have set the VM hub to modem mode and bought our own router or wireless mesh systems. You may be unwilling to spend the money, but in all honesty that is the best option. ISP provided hubs are built down to the lowest possible price, and VM's are amongst the crummiest.
If you do decide to spend some (more) money, then:
1) Avoid boosters - they're a kludge that can work, but doesn't always, and if you rent them from VM they're staggeringly poor value.
2) Consider a premium router from somebody like Asus (and their RT range) if you want highest possible wireless speeds....
3) ...or go with a mesh wifi system is you're prepared to trade a little bit of maximum speed for more comprehensive coverage round the property. And it's still be faster than the VM hub. I'm using a £100 TP-LInk Deco M4 mesh, and it's worlds better than the unreliable wifi of my Hub 3.
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You need to disable “smart” WiFi on the hub and split the bands
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