Is it true?
Yes and No, depending on your child's starting playing strength and the teaching ability of the GM/IM you can find.
If your child is already in Top 10 nationally of his/her age group, definitely you need a GM/IM coach.
If your child is above 1500, and is serious about developing his/her chess career further, YES, a GM/IM coach is very helpful.
If your child just starts learning chess, or doesn't know any tactics yet, or is still struggling under 1000, NO, you may not need a GM/IM at all.
GM/IM may have perfect knowledge, perfect opening lines, and perfect methods for solving problems perfectly. But these are not related to your child. Your child can't assimilate any of these perfect things. S/he will be overwhelmed, confused, and even discouraged. You don't feed a child or a very weak patient Ginseng, because his/her body can't take it. Too much nutrition and energy booster actually damage his/her health.
Most GM/IMs already forget how a beginner is thinking and learning. They will talk at a much higher level. They'd like your child to jump to that level right away. They don't understand why your child needs some steps in between, in their mind, it's so simple. And they may not know how to bridge the gap with these small steps. A GM/IM who can teach well at beginner level is a rare gem, if you are lucky to find one, grab him/her.
Nowadays, machines are much stronger than GMs and can calculate extremely accurate. For any position, they always give you the best move quickly. Even GMs are consulting the machines for better moves. Do you want to learn from a machine?
Now think about this: how many GM/IM's own children are playing well in chess? how many have the potential to become a GM/IM? Very few. Why? They have a GM/IM coach available at home 24x7.
GM is like a college professor, and IM an associate professor. Do you want a college professor to teach your child elementary school math?