The 1st reply is correct, the rule of thumb is that if you're flying on the airbus 321 or 320 we have 4 masks on odd numbered rows and 3 masks on even numbered rows, however, some of our aircraft have 4 masks available at all rows of seats(8 if you include both port and starboard sides). If there is space on the side opposite to the other half you may be able to sit closer, or if one seat is free on either of the rows with 4 masks, you will be able to sit together, ie 2 adults and 2 infants per 3 seats, plus you maybe able to put the infants down for a while during the flight. Either way the crew will be on hand to offer you assistance throughout the flight to warm bottles/food, and we have changing facilities available in most washrooms onboard our aircraft should you require this.
Also pushchairs are carried free of charge and can be used to carry the children all the way to the aircraft door (but on arrival will need to be collected at the outsized baggage carousel).
Baby food/drink is allowed to be taken through security too as long as it is kept to a minimal amount, however you may be asked to test it by tasting it at security check point at Manchester Terminal 2.
And just to let you know, you can ask these sorts of questions on monarchs official facebook page facebook.com/monarch and tweet @Monarch/
I hope this helps!