The website explains each symbol https://www.accesscard.online/how-it-works/
In the information about the +1 carers symbol it states:
This symbol is awarded to people for whom it would be significantly difficult to access services without support from another person.
You must be able to show that the support you need is substantial, significant or frequent, and directly related to your needs as a disabled person, and that without extra support you would receive a lesser service. It may mean an essential companion ticket, or it may mean a provider will offer additional support from their own staff. The support needed must exceed what could be reasonably expected of family/friends already accompanying you, and must be due to disability-related needs.
For children, we do not question that your child may need a certain amount of support when out and about, but we are unlikely to award the +1 symbol where they would already be in direct supervision and guidance by a parent or other family member.
So yes, it sounds like it wouldn’t be awarded for a young child who was at an age where children would typically be supported by parents anyway, assuming the support needs weren’t hugely above and beyond what a single parent were able to manage. I would hope there are situations where a child can be offered the symbol though, for example if the child had very complex medical needs or displayed a high level of challenging behaviour where they might require the support of two people or very close/ intensive support at all times.