Children should be learning aqida and the fundamentals of faith at a very young age. Around 1 to 2 years old is when kids should be learning about Allah and regularly referring to Him. How so?
Kids can understand basic concepts like: Allah is the One that provides for us. All the good things we enjoy like toys and ice cream, etc., or anything pretty or beautiful we see are from Allah. We should thank Allah for these things. If your children love dinosaurs or trucks or dolls, then connect that to Allah as well.
Tell your kids about Allah being loving and protecting us and taking care of us. The masjid is also the house of Allah, which we should regularly visit. When we want to speak to Allah, we pray (salah) and when we want to listen to Him, we recite Quran. Also, we love the Messenger of Allah ﷺ and we send him salawat whenever we can. Kids love to imitate their parents, so as long as mom and dad are doing it, it’s like fun and enjoyment for the rugrats.
You also have to pick the right moments to talk to your kids about their Creator. The best times are when they are happy and having fun. Whenever it is time for ice cream, for example, we make sure to thank Allah (“Thank you Allah!”) and say bismillah, etc. Ice cream time thus becomes a time for dhikr and gratitude. For a 2 or 3 year old, ice cream time is the height of joy in life, so it’s important to pair these times with remembering and mentioning Allah. Bed time is also a perfect opportunity because that’s when the kids hang on to a parent’s every word (mostly because they want to delay sleep, that’s their “trick” so take advantage of it).
Also, around age 3.5 or 4, concepts like tawhid are comprehensible, i.e., that Allah is One, He has no partner, there is nothing like Him, etc. Of course, kids will learn things like the shahada and surat al-ikhlas well before that, but they won’t be able to grasp the meaning until around 4 years old.
By speaking to our kids about Allah and ingraining these core concepts at an early age, this will bi idhnillah have a huge impact on their faith later in life. May Allah make our children of the salihin!