disclaimer: i am not the most amazing modder in the world. my knowledge is limited, but this is based on what i have gleaned.
grox eggs should hatch in something like 3-6 months at a guess (that's the normal time for any egg to hatch, based on latest !!SCIENCE!! threads i've seen). Grox should take 2 years to grow up (that's what "[Child:2] means". in the raws, it appeared you had another tag (the name for grox children) on the same line instead of on the next line and at the first tab, but i doubt that's the problem. most likely problems causing eggs not to hatch:
1) eggs removed from nest box for any reason, no matter how briefly (for example, to move them to a food stockpile which accepts eggs, or to use them for cooking).
2) mother leaves the nest box for any reason, no matter how briefly (for example, to go kill a vermin). i suspect this is likely your main possibility. try locking the nest box behind a tightly-closed door with no room to move off the box once the grox goes in.
3) something large (ie dwarf-sized... probably hiver-sized as well) other than the mother stands on the nest box. some !!SCIENCE!! shows that animal trainers who are coming to reinforce training *can* stand on the next box without preventing eggs from hatching, though i've not personally tested this to be sure (and i only saw one account, nobody had verified yet). general wisdom before was that this, much like the previous two, applies no matter how briefly the event occurs. it is also possible that the size threshold is determined by the size of the creature stepping on the box compared to the size of the egg or the size of the egglaying creature. more !!SCIENCE!! is required to know for sure.
4) you have too many of that creature type (or creatures in general) on the map at the moment (i consider this unlikely, but don't know your exact situation).
5) the egg simply didn't get fertilized. the RNG hates you, and there's not an awful lot you can do about it except to get rid of the eggs and hope it works out better next time. unless of course you didn't bring a male, in which case, well... the problem should be obvious.
vent lurkers *do* lay eggs, but *don't* have children if i'm not mistaken. you would need to mod the child tag in. without [Child:<number>] to tell the game how long it takes for a child to reach adulthood, you simply don't get children.
also of note, currently zombies will have children. not sure if that's intended.