You can use the same envelope for both applications. Just to be safe, write a separate cheque for each application, and paperclip (in several locations) each application to keep it separate from the other.
BUT FIRST, how old is your child? He/she has to be 18 or older to apply for citizenship. If the child is younger than that, has a GC and is in your and/or your wife's custody, then the child will automatically become a citizen if your wife takes her oath before the child's 18th birthday. The child might also already be a citizen through you.