There is nothing wrong with starting small. Many church's cannot afford to start at all if they don't start small. Please start, even if its small. Most dollar stores have fidgets and sensory toys like play dough, squishy toys, and spinners. Walmart is now selling weighted blankets and noise canceling headphones as well as the items above. It really is very simple to begin. Here's my go to advice for beginners.

Pray over it. 

Start with love. Accept all people as they are, the image bearers of God. Start with a desire to find volunteers who will see that too. Educate your church on disabilities. Ask someone who has experience to come and speak to your church. I would be happy to set up a meeting to help you do so, or even preach a sermon on inclusion one Sunday, so contact me below if your interested. 

Create sensory bags for individuals with disabilities. These should be marked with the users name to avoid anxiety in later dates. Inside include: 2-3 different kinds of fidgets, noise canceling head phones, and a set of communication cards. If the children will be attending during church hours I encourage you to include a visual schedule as well(see below).
 
Encourage peers to help each other. Neurotypical peers cannot be solely responsible for impacted children but if you can have 2-4 responsible "friends" per an age group it will go a long way to making this easier for your volunteers during service times.  Make sure to discuss with parents first. 

Address the friends disability in the same way they themselves, or their family address it. Some families prefer person first language like "Johnny who has Down Syndrome" and others, like my own children, may proudly identify using their disability. My J will tell you "I am autistic. I cannot take off my autism so I am not a person with autism." 

Feel free to contact me for more suggestions below.