If a mother is in isolation but the child comes and touches her, again
goes and touch the father, grand father\mother etc will it contaminate
them?
NO, because of the following verses:
Wild animals killed by dogs; a fruit made to fall by a bird; what has
been passed around by children or handled by women; goods dis- played
for sale so long as they have not been spoilt by gnats and flies
landing on them; impurities in the mouths of women; water collected
on the ground sufficient to slake the thirst of cows––having
enumerated all these, Prajapati declared them to be pure.
VDS 3.54
Before his initiation, a child may behave, speak, and eat as he
pleases. He may not partake of ritual offerings and should observe
chastity. He may void urine and excrement whenever he has the urge.
The set of rules regarding sipping water and other rituals of
purification do not apply to him, other than wiping, washing, and
sprinkling with water, and no one is made impure by his touch. No
one should ever employ him to make fire oblations or Bali offerings,
and he should not be made to recite the Veda, except for uttering
‘Svadha¯’.
GDS 2.24
Also, in general, rules of Dharma, like purity-impurity etc, seems not to apply to children until they get to the age of initiation (which varies with one's caste; like for Brahmins it's 8 from the time of conception not birth etc)
Children do not become impure until the first feeding with solid food
has taken place; according to some, until they have completed one
year; or until they are able or until they have under-to distinguish
the cardinal points; for it is through gone initiation, which is the
superior position, this that they come under the jurisdiction of the
scriptures. This last view is the authoritative one, and it is the
traditional teaching.*
ADS 2.16.1
Yet another relevant verse is the following:
One’s own bed, seat, clothes, wife, children, and water pot are pure
with respect to oneself; but they are impure vis-à-vis others.
BDS 1.98
So, in short, children can be considered as pure in all circumstances.
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Abbreviations used:
VDS- The Dharmasutras of Vashishta.
GDS- The same of Gautama.
ADS- The same of Apasthamba.
BDS- The same of Baudhayana.