Early childhood mental health refers to children from birth to five years old.  If your children don't even have their baby teeth yet there are other indicators of mental health concerns and It's very crucial for parents to identify, manage and express their emotions in healthy ways.
For young kids mental health It's mainly about doing the small things consistently like responding to their smiles or cries, comforting them when they're upset or scared, and simply talking to them.  These habits help babies develop trusting relationships with their parents, allowing them to support their overall mental health.
Some great indicators of healthy and secure attachment relationships include if your child comes you when they need help, comfort, get hurt, that kids show affection, greets the caregiver, engage with you when their out playing, and are more comfortable with their caregivers then with strangers.
Some clear signs to watch out for include if your child has problems sleeping, eating, they over or under respond to things around them, or they seem very distant from you or other caregivers.  Some stressors for kids after birth can also Impact their mental health like a new baby or constantly changing homes can impact how your kids behave.
Some good tips for parents to increase attachment with their kids include making eye contact, singing, smiling, talking, laughing, providing basic needs, giving them a lot of attention, responding to their needs, and taking care of your own physical and mental health.  This last one is often overlooked but kids get stressed out when their parents who they often idolize as superheroes are struggling with physical or mental illnesses.