The child had an unnatural exposer to high amounts of Bijuu chakra while their own chakra pathway system was forming, this results in a phenomenon whereby a human is born with bijuu chakra instead of normal chakra, though they cannot transform, use tailed beast skills or have a demon extracted from them like normal holders of Bijuu.
They do have the effect of having had the demons chakra "rub off on them", for example, a Bijuu child that has had the 4 tails as a "parent" would have access to lava chakra with out a kekkei genkai, theyre chakra reserves are nowhere near as massive as a real Bijuu but have increased segnificantly from normal ninja of their age. They enjoy an accelerated healing rate, though not as much as a real Bijuu and by no means regeneration, merely shortening the recovery time needed between battles.
Bijuu Kids can tap into demonic chakra increasing strength and speed along with their natural elemental power. Another advantage is their Kekkei Tota which is three basic elements a long with two advanced. This demonic chakra increases all stats by one (primary becomes super primary secondary becomes primary). There are disadvantages how ever when you lose Demonic Form your stats drop one level primary becomes secondary ect ect. Also upon exit you can't move for 3 post after your no longer influenced and lastly. Their chakra reserves are one rank above their normal rank. A genin has chuunin reserves.