Mixed dentition stage

Why is the newly erupted front teeth serrated on the edge?

It is normal for newly erupted front teeth to appear saw-shaped on the edge. It will be naturally grinded and smoothened after a period of time.
My child's upper front teeth are flared with a gap in between. Does he/she need orthodontic treatment?

This is a transitional period called the "Ugly Duckling" stage. Usually, when the upper canines on both sides erupt, the front teeth will straighten and the gap will close up.
My child's lower permanent front tooth erupted behind the deciduous front tooth. Is it necessary to extract the deciduous front tooth?

Commonly, the deciduous tooth will become loose by itself. The tongue will push the permanent front tooth forward into position. Gradually, the deciduous tooth will exfoliate by itself. The permanent tooth will align itself into normal position. Therefore, it usually do not need to remove the deciduous tooth in this kind of situation.
The newly erupted permanent front teeth are not well aligned. Can the neighbouring deciduous teeth be extracted to allow front teeth to align correctly?

Since permanent teeth are bigger than deciduous teeth, and jawbones are not yet completely developed in children, the newly erupted permanent teeth may not have enough room to align properly and they look crowded. However, we usually cannot determine whether the permanent teeth will have a good alignment until all the deciduous teeth are exfoliated and replaced by permanent teeth. If you have any queries, please ask your dentist for further advice.
My child's has irregular teeth. When is the suitable time for him/her to have orthodontic treatment?

Generally speaking, we cannot be sure whether the tooth alignment is irregular or not until all the permanent teeth have replaced the deciduous teeth and the development of jawbones have stabilized. Then, we could decide whether an orthodontic treatment is needed.
Is there a problem if the transition from deciduous to permanent teeth happens later in my child than his/her friends of the same age?

You don't need to worry since the timing of tooth transition varies between different individuals. If you have any queries regarding tooth transition, you may consult your dentist.