Little feet need lots of wiggle room to grow properly. Measuring their feet every 2-3 months is typical.
The shoe store is most likely selling you a size 7 so there is enough wiggle room AND growth room. Otherwise you will be buying a new pair of shoes every month!
Also, be sure to get shoes that flex. Barefoot is best for developing feet, but flexible shoes are good for when they need their feet protected.
Where we bought our daughters shoes, they told us about a thumb width is what you are looking for in 'extra room'. That is consistent with other information I've been able to look up. So, buy a pair with the thumb width and then when the toes are getting up near the top... buy a new pair. You don't want to be squeezing their little feet into shoes... that is where developmental issues come into play.
Hope this helps! Sounds like you have your hands full with a 22 month old... and you say '4 month old boyS' ... i'm guessing twins?? oiy!