2. Solutions.
The best solution is to keep children away from that place and try not to let them in, because their resistance is weak, even if they breathe in a little, it will have adverse effects.
To take a step back, you can find a professional (must be professional) indoor air purification company to deal with it (because the treatment process is also removed by chemical means, unprofessional is likely to cause secondary pollution)
Or you can make diatom mud on the wall (a new decorative material, in addition to formaldehyde, etc. , remember to choose non-adhesive)
Then you should pay attention to placing some plants at ordinary times.